Python Introduction
What is Python?
Python is a popular programming language. It was created by Guido van Rossum, and released in 1991.
It is used for:
- web development (server-side),
- software development,
- mathematics,
- system scripting.
What can Python do?
- Python can be used on a server to create web applications.
- Python can be used alongside software to create workflows.
- Python can connect to database systems. It can also read and modify files.
- Python can be used to handle big data and perform complex mathematics.
- Python can be used for rapid prototyping, or for production-ready software development.
Why Python?
- Python works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc).
- Python has a simple syntax similar to the English language.
- Python has syntax that allows developers to write programs with fewer lines than some other programming languages.
- Python runs on an interpreter system, meaning that code can be executed as soon as it is written. This means that prototyping can be very quick.
- Python can be treated in a procedural way, an object-oriented way or a functional way.
Good to know
- The most recent major version of Python is Python 3, which we shall be using in this tutorial. However, Python 2, although not being updated with anything other than security updates, is still quite popular.
- In this tutorial Python will be written in a text editor. It is possible to write Python in an Integrated Development Environment, such as Thonny, Pycharm, Netbeans or Eclipse which are particularly useful when managing larger collections of Python files.
Python Syntax compared to other programming languages
- Python was designed for readability, and has some similarities to the English language with influence from mathematics.
- Python uses new lines to complete a command, as opposed to other programming languages which often use semicolons or parentheses.
- Python relies on indentation, using whitespace, to define scope; such as the scope of loops, functions and classes. Other programming languages often use curly-brackets for this purpose.
Example
print(“Hello, World!”)
Python Getting Started
Python Install
Many PCs and Macs will have python already installed.
To check if you have python installed on a Windows PC, search in the start bar for Python or run the following on the Command Line (cmd.exe):
C:\Users\Your Name>python –version
To check if you have python installed on a Linux or Mac, then on linux open the command line or on Mac open the Terminal and type:
python –version
If you find that you do not have Python installed on your computer, then you can download it for free from the following website: https://www.python.org/
Python Quickstart
Python is an interpreted programming language, this means that as a developer you write Python (.py) files in a text editor and then put those files into the python interpreter to be executed.
The way to run a python file is like this on the command line:
C:\Users\Your Name>python helloworld.py
Where “helloworld.py” is the name of your python file.
Let’s write our first Python file, called helloworld.py, which can be done in any text editor.
helloworld.py
print(“Hello, World!”)
Simple as that. Save your file. Open your command line, navigate to the directory where you saved your file, and run:
C:\Users\Your Name>python helloworld.py
The output should read:
Hello, World!
Congratulations, you have written and executed your first Python program.
The Python Command Line
To test a short amount of code in python sometimes it is quickest and easiest not to write the code in a file. This is made possible because Python can be run as a command line itself.
Type the following on the Windows, Mac or Linux command line:
C:\Users\Your Name>python
Or, if the “python” command did not work, you can try “py”:
C:\Users\Your Name>py
From there you can write any python, including our hello world example from earlier in the tutorial:
C:\Users\Your Name>python
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type “help”, “copyright”, “credits” or “license” for more information.
>>> print(“Hello, World!”)
Which will write “Hello, World!” in the command line:
C:\Users\Your Name>python
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type “help”, “copyright”, “credits” or “license” for more information.
>>> print(“Hello, World!”)
Hello, World!
Whenever you are done in the python command line, you can simply type the following to quit the python command line interface:
exit()
Python Syntax
Execute Python Syntax
As we learned in the previous page, Python syntax can be executed by writing directly in the Command Line:
>>> print(“Hello, World!”)
Hello, World!
By creating a python file on the server, using the .py file extension, and running it in the Command Line:
C:\Users\Your Name>python myfile.py
Python Indentation
Indentation refers to the spaces at the beginning of a code line.
Where in other programming languages the indentation in code is for readability only, the indentation in Python is very important.
Python uses indentation to indicate a block of code.
Example
if 5 > 2:
print(“Five is greater than two!”)
Python will give you an error if you skip the indentation:
Example
Syntax Error:
if 5 > 2:
print(“Five is greater than two!”)
The number of spaces is up to you as a programmer, the most common use is four, but it has to be at least one.
Example
if 5 > 2:
print(“Five is greater than two!”)
if 5 > 2:
print(“Five is greater than two!”)
You have to use the same number of spaces in the same block of code, otherwise Python will give you an error:
Example
Syntax Error:
if 5 > 2:
print(“Five is greater than two!”)
print(“Five is greater than two!”)
Python Variables
In Python, variables are created when you assign a value to it:
Example
Variables in Python:
x = 5
y = “Hello, World!”
Python has no command for declaring a variable.
Comments
Python has commenting capability for the purpose of in-code documentation.
Comments start with a #, and Python will render the rest of the line as a comment:
Example
Comments in Python:
#This is a comment.
print(“Hello, World!”)
Python Comments
Comments can be used to explain Python code.
Comments can be used to make the code more readable.
Comments can be used to prevent execution when testing code.
Creating a Comment
Comments starts with a #
, and Python will ignore them:
Example
#This is a comment
print(“Hello, World!”)
Comments can be placed at the end of a line, and Python will ignore the rest of the line:
Example
print(“Hello, World!”) #This is a comment
A comment does not have to be text that explains the code, it can also be used to prevent Python from executing code:
Example
#print(“Hello, World!”)
print(“Cheers, Mate!”)
Multiline Comments
Python does not really have a syntax for multiline comments.
To add a multiline comment you could insert a #
for each line:
Example
#This is a comment
#written in
#more than just one line
print(“Hello, World!”)
Or, not quite as intended, you can use a multiline string.
Since Python will ignore string literals that are not assigned to a variable, you can add a multiline string (triple quotes) in your code, and place your comment inside it:
Example
“””
This is a comment
written in
more than just one line
““”
print(“Hello, World!”)
Python Variables
Variables
Variables are containers for storing data values.
Creating Variables
Python has no command for declaring a variable.
A variable is created the moment you first assign a value to it.
Example
x = 5
y = “John”
print(x)
print(y)
Variables do not need to be declared with any particular type, and can even change type after they have been set.
Example
x = 4 # x is of type int
x = “Sally” # x is now of type str
print(x)
Casting
If you want to specify the data type of a variable, this can be done with casting.
Example
x = str(3) # x will be ‘3’
y = int(3) # y will be 3
z = float(3) # z will be 3.0
Get the Type
You can get the data type of a variable with the type()
function.
Example
x = 5
y = “John”
print(type(x))
print(type(y))
Single or Double Quotes?
String variables can be declared either by using single or double quotes:
Example
x = “John”
# is the same as
x = ‘John’
Case-Sensitive
Variable names are case-sensitive.
Example
This will create two variables:
a = 4
A = “Sally”
#A will not overwrite a
Python – Variable Names
Variable Names
A variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume). Rules for Python variables:
- A variable name must start with a letter or the underscore character
- A variable name cannot start with a number
- A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
- Variable names are case-sensitive (age, Age and AGE are three different variables)
- A variable name cannot be any of the Python keywords.
Example
Legal variable names:
myvar = “John”
my_var = “John”
_my_var = “John”
myVar = “John”
MYVAR = “John”
myvar2 = “John”
Example
Illegal variable names:
2myvar = “John”
my-var = “John”
my var = “John”
Python Variables – Assign Multiple Values
Many Values to Multiple Variables
Python allows you to assign values to multiple variables in one line:
Example
x, y, z = “Orange”, “Banana”, “Cherry”
print(x)
print(y)
print(z)
Note: Make sure the number of variables matches the number of values, or else you will get an error.
One Value to Multiple Variables
And you can assign the same value to multiple variables in one line:
Example
x = y = z = “Orange”
print(x)
print(y)
print(z)
Unpack a Collection
If you have a collection of values in a list, tuple etc. Python allows you to extract the values into variables. This is called unpacking.
Example
Unpack a list:
fruits = [“apple”, “banana”, “cherry”]
x, y, z = fruits
print(x)
print(y)
print(z)
Python – Output Variables
Output Variables
The Python print()
function is often used to output variables.
Example
x = “Python is awesome”
print(x)
In the print()
function, you output multiple variables, separated by a comma:
Example
x = “Python”
y = “is”
z = “awesome”
print(x, y, z)
You can also use the +
operator to output multiple variables:
Example
x = “Python “
y = “is “
z = “awesome”
print(x + y + z)
Notice the space character after "Python "
and "is "
, without them the result would be “Pythonisawesome”.
For numbers, the +
character works as a mathematical operator:
Example
x = 5
y = 10
print(x + y)
In the print()
function, when you try to combine a string and a number with the +
operator, Python will give you an error:
Example
x = 5
y = “John”
print(x + y)
The best way to output multiple variables in the print()
function is to separate them with commas, which even support different data types:
Example
x = 5
y = “John”
print(x, y)
Python – Global Variables
Global Variables
Variables that are created outside of a function (as in all of the examples above) are known as global variables.
Global variables can be used by everyone, both inside of functions and outside.
Example
Create a variable outside of a function, and use it inside the function
x = “awesome”
def myfunc():
print(“Python is “ + x)
myfunc()
If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the function. The global variable with the same name will remain as it was, global and with the original value.
Example
Create a variable inside a function, with the same name as the global variable
x = “awesome”
def myfunc():
x = “fantastic”
print(“Python is “ + x)
myfunc()
print(“Python is “ + x)
The global Keyword
Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function.
To create a global variable inside a function, you can use the global
keyword.
Example
If you use the global
keyword, the variable belongs to the global scope:
def myfunc():
global x
x = “fantastic”myfunc()
print(“Python is “ + x)
Also, use the global
keyword if you want to change a global variable inside a function.
Example
To change the value of a global variable inside a function, refer to the variable by using the global
keyword:
x = “awesome”
def myfunc():
global x
x = “fantastic”
myfunc()
print(“Python is “ + x)
Python – Variable Exercises
Test Yourself With Exercises
Now you have learned a lot about variables, and how to use them in Python.
Are you ready for a test?
Try to insert the missing part to make the code work as expected:
Python Data Types
Built-in Data Types
In programming, data type is an important concept.
Variables can store data of different types, and different types can do different things.
Python has the following data types built-in by default, in these categories:
Text Type: | str |
Numeric Types: | int , float , complex |
Sequence Types: | list , tuple , range |
Mapping Type: | dict |
Set Types: | set , frozenset |
Boolean Type: | bool |
Binary Types: | bytes , bytearray , memoryview |
None Type: | NoneType |
Getting the Data Type
You can get the data type of any object by using the type()
function:
Example
Print the data type of the variable x:
x = 5
print(type(x))
Setting the Data Type
In Python, the data type is set when you assign a value to a variable:
Example | Data Type | Num |
---|---|---|
x = “Hello World” | str | 1 |
x = 20 | int | 2 |
x = 20.5 | float | 3 |
x = 1j | complex | 4 |
x = [“apple”, “banana”, “cherry”] | list | 5 |
x = (“apple”, “banana”, “cherry”) | tuple | 6 |
x = range(6) | range | 7 |
x = {“name” : “John”, “age” : 36} | dict | 8 |
x = {“apple”, “banana”, “cherry”} | set | 9 |
x = frozenset({“apple”, “banana”, “cherry”}) | frozenset | 10 |
x = True | bool | 11 |
x = b”Hello” | bytes | 12 |
x = bytearray(5) | bytearray | 13 |
x = memoryview(bytes(5)) | memoryview | 14 |
x = None | NoneType | 15 |
Setting the Specific Data Type
If you want to specify the data type, you can use the following constructor functions:
Example | Data Type | Num |
---|---|---|
x = str(“Hello World”) | str | 1 |
x = int(20) | int | 2 |
x = float(20.5) | float | 3 |
x = complex(1j) | complex | 4 |
x = list((“apple”, “banana”, “cherry”)) | list | 5 |
x = tuple((“apple”, “banana”, “cherry”)) | tuple | 6 |
x = range(6) | range | 7 |
x = dict(name=”John”, age=36) | dict | 8 |
x = set((“apple”, “banana”, “cherry”)) | set | 9 |
x = frozenset((“apple”, “banana”, “cherry”)) | frozenset | 10 |
x = bool(5) | bool | 11 |
x = bytes(5) | bytes | 12 |
x = bytearray(5) | bytearray | 13 |
x = memoryview(bytes(5)) | memoryview | 14 |
Python Numbers
Python Numbers
There are three numeric types in Python:
int
float
complex
Variables of numeric types are created when you assign a value to them:
Example
x = 1 # int
y = 2.8 # float
z = 1j # complex
To verify the type of any object in Python, use the type()
function:
Example
print(type(x))
print(type(y))
print(type(z))
Int
Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.
Example
Integers:
x = 1
y = 35656222554887711
z = –3255522print(type(x))
print(type(y))
print(type(z))
Float
Float, or “floating point number” is a number, positive or negative, containing one or more decimals.
Example
Floats:
x = 1.10
y = 1.0
z = –35.59print(type(x))
print(type(y))
print(type(z))
Float can also be scientific numbers with an “e” to indicate the power of 10.
Example
Floats:
x = 35e3
y = 12E4
z = –87.7e100print(type(x))
print(type(y))
print(type(z))
Complex
Complex numbers are written with a “j” as the imaginary part:
Example
Complex:
x = 3+5j
y = 5j
z = -5jprint(type(x))
print(type(y))
print(type(z))
Type Conversion
You can convert from one type to another with the int()
, float()
, and complex()
methods:
Example
Convert from one type to another:
x = 1 # int
y = 2.8 # float
z = 1j # complex
#convert from int to float:
a = float(x)#convert from float to int:
b = int(y)
#convert from int to complex:
c = complex(x)
print(a)
print(b)
print(c)
print(type(a))
print(type(b))
print(type(c))
Note: You cannot convert complex numbers into another number type.
Random Number
Python does not have a random()
function to make a random number, but Python has a built-in module called random
that can be used to make random numbers:
Example
Import the random module, and display a random number between 1 and 9:
import random
print(random.randrange(1, 10))
Python Casting
Specify a Variable Type
There may be times when you want to specify a type on to a variable. This can be done with casting. Python is an object-orientated language, and as such it uses classes to define data types, including its primitive types.
Casting in python is therefore done using constructor functions:
- int() – constructs an integer number from an integer literal, a float literal (by removing all decimals), or a string literal (providing the string represents a whole number)
- float() – constructs a float number from an integer literal, a float literal or a string literal (providing the string represents a float or an integer)
- str() – constructs a string from a wide variety of data types, including strings, integer literals and float literals
Example
Integers:
x = int(1) # x will be 1
y = int(2.8) # y will be 2
z = int(“3”) # z will be 3
Example
Floats:
x = float(1) # x will be 1.0
y = float(2.8) # y will be 2.8
z = float(“3”) # z will be 3.0
w = float(“4.2”) # w will be 4.2
Example
Strings:
x = str(“s1”) # x will be ‘s1’
y = str(2) # y will be ‘2’
z = str(3.0) # z will be ‘3.0’
Python Strings
Strings
Strings in python are surrounded by either single quotation marks, or double quotation marks.
‘hello’ is the same as “hello”.
You can display a string literal with the print()
function:
Example
print(“Hello”)
print(‘Hello’)
Assign String to a Variable
Assigning a string to a variable is done with the variable name followed by an equal sign and the string:
Example
a = “Hello”
print(a)
Multiline Strings
You can assign a multiline string to a variable by using three quotes:
Example
You can use three double quotes:
a = “”“Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.”“”
print(a)
Or three single quotes:
Example
a = ”‘Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.’”
print(a)
Note: in the result, the line breaks are inserted at the same position as in the code.
Strings are Arrays
Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters.
However, Python does not have a character data type, a single character is simply a string with a length of 1.
Square brackets can be used to access elements of the string.
Example
Get the character at position 1 (remember that the first character has the position 0):
a = “Hello, World!”
print(a[1])
Looping Through a String
Since strings are arrays, we can loop through the characters in a string, with a for
loop.
Example
Loop through the letters in the word “banana”:
for x in “banana”:
print(x)
Learn more about For Loops in our Python For Loops chapter.
String Length
To get the length of a string, use the len()
function.
Example
The len()
function returns the length of a string:
a = “Hello, World!”
print(len(a))
Check String
To check if a certain phrase or character is present in a string, we can use the keyword in
.
Example
Check if “free” is present in the following text:
txt = “The best things in life are free!”
print(“free” in txt)
Use it in an if
statement:
Example
Print only if “free” is present:
txt = “The best things in life are free!”
if “free” in txt:
print(“Yes, ‘free’ is present.”)
Check if NOT
To check if a certain phrase or character is NOT present in a string, we can use the keyword not in
.
Example
Check if “expensive” is NOT present in the following text:
txt = “The best things in life are free!”
print(“expensive” not in txt)
Use it in an if
statement:
Example
print only if “expensive” is NOT present:
txt = “The best things in life are free!”
if “expensive” not in txt:
print(“No, ‘expensive’ is NOT present.”)
Python – Slicing Strings
Slicing
You can return a range of characters by using the slice syntax.
Specify the start index and the end index, separated by a colon, to return a part of the string.
Example
Get the characters from position 2 to position 5 (not included):
b = “Hello, World!”
print(b[2:5])
Note: The first character has index 0.
Slice From the Start
By leaving out the start index, the range will start at the first character:
Example
Get the characters from the start to position 5 (not included):
b = “Hello, World!”
print(b[:5])
Slice To the End
By leaving out the end index, the range will go to the end:
Example
Get the characters from position 2, and all the way to the end:
b = “Hello, World!”
print(b[2:])
Negative Indexing
Use negative indexes to start the slice from the end of the string:
Example
Get the characters:
From: “o” in “World!” (position -5)
To, but not included: “d” in “World!” (position -2):
b = “Hello, World!”
print(b[-5:-2])
Python – Modify Strings
Python has a set of built-in methods that you can use on strings.
Upper Case
Example
The upper()
method returns the string in upper case:
a = “Hello, World!”
print(a.upper())
Lower Case
Example
The lower()
method returns the string in lower case:
a = “Hello, World!”
print(a.lower())
Remove Whitespace
Whitespace is the space before and/or after the actual text, and very often you want to remove this space.
Example
The strip()
method removes any whitespace from the beginning or the end:
a = ” Hello, World! “
print(a.strip()) # returns “Hello, World!”
Replace String
Example
The replace()
method replaces a string with another string:
a = “Hello, World!”
print(a.replace(“H”, “J”))
Split String
The split()
method returns a list where the text between the specified separator becomes the list items.
Example
The split()
method splits the string into substrings if it finds instances of the separator:
a = “Hello, World!”
print(a.split(“,”)) # returns [‘Hello’, ‘ World!’]
Python – String Concatenation
String Concatenation
To concatenate, or combine, two strings you can use the + operator.
Example
Merge variable a
with variable b
into variable c
:
a = “Hello”
b = “World”
c = a + b
print(c)
Example
To add a space between them, add a " "
:
a = “Hello”
b = “World”
c = a + ” “ + b
print(c)
Python – Format – Strings
String Format
As we learned in the Python Variables chapter, we cannot combine strings and numbers like this:
Example
age = 36
txt = “My name is John, I am “ + age
print(txt)
But we can combine strings and numbers by using the format()
method!
The format()
method takes the passed arguments, formats them, and places them in the string where the placeholders {}
are:
Example
Use the format()
method to insert numbers into strings:
age = 36
txt = “My name is John, and I am {}”
print(txt.format(age))
The format() method takes unlimited number of arguments, and are placed into the respective placeholders:
Example
quantity = 3
itemno = 567
price = 49.95
myorder = “I want {} pieces of item {} for {} dollars.”
print(myorder.format(quantity, itemno, price))
You can use index numbers {0}
to be sure the arguments are placed in the correct placeholders:
Example
quantity = 3
itemno = 567
price = 49.95
myorder = “I want to pay {2} dollars for {0} pieces of item {1}.”
print(myorder.format(quantity, itemno, price))
Python – Escape Characters
Escape Character
To insert characters that are illegal in a string, use an escape character.
An escape character is a backslash \
followed by the character you want to insert.
An example of an illegal character is a double quote inside a string that is surrounded by double quotes:
Example
You will get an error if you use double quotes inside a string that is surrounded by double quotes:
txt = “We are the so-called “Vikings” from the north.”
To fix this problem, use the escape character \"
:
Example
The escape character allows you to use double quotes when you normally would not be allowed:
txt = “We are the so-called \”Vikings\” from the north.”
Escape Characters
Other escape characters used in Python:
Code | Result | Num |
---|---|---|
\’ | Single Quote | 1 |
\\ | Backslash | 2 |
\n | New Line | 3 |
\r | Carriage Return | 4 |
\t | Tab | 5 |
\b | Backspace | 6 |
\f | Form Feed | 7 |
\ooo | Octal value | 8 |
\xhh | Hex value |
Python – String Methods
String Methods
Python has a set of built-in methods that you can use on strings.
Note: All string methods return new values. They do not change the original string.
Method | Description |
---|---|
capitalize() | Converts the first character to upper case |
casefold() | Converts string into lower case |
center() | Returns a centered string |
count() | Returns the number of times a specified value occurs in a string |
encode() | Returns an encoded version of the string |
endswith() | Returns true if the string ends with the specified value |
expandtabs() | Sets the tab size of the string |
find() | Searches the string for a specified value and returns the position of where it was found |
format() | Formats specified values in a string |
format_map() | Formats specified values in a string |
index() | Searches the string for a specified value and returns the position of where it was found |
isalnum() | Returns True if all characters in the string are alphanumeric |
isalpha() | Returns True if all characters in the string are in the alphabet |
isdecimal() | Returns True if all characters in the string are decimals |
isdigit() | Returns True if all characters in the string are digits |
isidentifier() | Returns True if the string is an identifier |
islower() | Returns True if all characters in the string are lower case |
isnumeric() | Returns True if all characters in the string are numeric |
isprintable() | Returns True if all characters in the string are printable |
isspace() | Returns True if all characters in the string are whitespaces |
istitle() | Returns True if the string follows the rules of a title |
isupper() | Returns True if all characters in the string are upper case |
join() | Joins the elements of an iterable to the end of the string |
ljust() | Returns a left justified version of the string |
lower() | Converts a string into lower case |
lstrip() | Returns a left trim version of the string |
maketrans() | Returns a translation table to be used in translations |
partition() | Returns a tuple where the string is parted into three parts |
replace() | Returns a string where a specified value is replaced with a specified value |
rfind() | Searches the string for a specified value and returns the last position of where it was found |
rindex() | Searches the string for a specified value and returns the last position of where it was found |
rjust() | Returns a right justified version of the string |
rpartition() | Returns a tuple where the string is parted into three parts |
rsplit() | Splits the string at the specified separator, and returns a list |
rstrip() | Returns a right trim version of the string |
split() | Splits the string at the specified separator, and returns a list |
splitlines() | Splits the string at line breaks and returns a list |
startswith() | Returns true if the string starts with the specified value |
strip() | Returns a trimmed version of the string |
swapcase() | Swaps cases, lower case becomes upper case and vice versa |
title() | Converts the first character of each word to upper case |
translate() | Returns a translated string |
upper() | Converts a string into upper case |
zfill() | Fills the string with a specified number of 0 values at the beginning |
Python – String Exercises
Test Yourself With Exercises
Now you have learned a lot about Strings, and how to use them in Python.
Are you ready for a test?
Try to insert the missing part to make the code work as expected:
Python Booleans
Booleans represent one of two values: True
or False
.
Boolean Values
In programming you often need to know if an expression is True
or False
.
You can evaluate any expression in Python, and get one of two answers, True
or False
.
When you compare two values, the expression is evaluated and Python returns the Boolean answer:
Example
print(10 > 9)
print(10 == 9)
print(10 < 9)
When you run a condition in an if statement, Python returns True
or False
:
Example
Print a message based on whether the condition is True
or False
:
a = 200
b = 33if b > a:
print(“b is greater than a”)
else:
print(“b is not greater than a”)
Evaluate Values and Variables
The bool()
function allows you to evaluate any value, and give you True
or False
in return,
Example
Evaluate a string and a number:
print(bool(“Hello”))
print(bool(15))
Example
Evaluate two variables:
x = “Hello”
y = 15print(bool(x))
print(bool(y))
Most Values are True
Almost any value is evaluated to True
if it has some sort of content.
Any string is True
, except empty strings.
Any number is True
, except 0
.
Any list, tuple, set, and dictionary are True
, except empty ones.
Example
The following will return True:
bool(“abc”)
bool(123)
bool([“apple”, “cherry”, “banana”])
Some Values are False
In fact, there are not many values that evaluate to False
, except empty values, such as ()
, []
, {}
, ""
, the number 0
, and the value None
. And of course the value False
evaluates to False
.
Example
The following will return False:
bool(False)
bool(None)
bool(0)
bool(“”)
bool(())
bool([])
bool({})
One more value, or object in this case, evaluates to False
, and that is if you have an object that is made from a class with a __len__
function that returns 0
or False
:
Example
class myclass():
def __len__(self):
return 0myobj = myclass()
print(bool(myobj))
Functions can Return a Boolean
You can create functions that returns a Boolean Value:
Example
Print the answer of a function:
def myFunction() :
return Trueprint(myFunction())
You can execute code based on the Boolean answer of a function:
Example
Print “YES!” if the function returns True, otherwise print “NO!”:
def myFunction() :
return Trueif myFunction():
print(“YES!”)
else:
print(“NO!”)
Python also has many built-in functions that return a boolean value, like the isinstance()
function, which can be used to determine if an object is of a certain data type:
Example
Check if an object is an integer or not:
x = 200
print(isinstance(x, int))
Python Operators
Python Operators
Operators are used to perform operations on variables and values.
In the example below, we use the +
operator to add together two values:
Example
print(10 + 5)
Python divides the operators in the following groups:
- Arithmetic operators
- Assignment operators
- Comparison operators
- Logical operators
- Identity operators
- Membership operators
- Bitwise operators
Python Arithmetic Operators
Arithmetic operators are used with numeric values to perform common mathematical operations:
Operator | Name | Example | Num |
---|---|---|---|
+ | Addition | x + y | 1 |
– | Subtraction | x – y | 2 |
* | Multiplication | x * y | 3 |
/ | Division | x / y | 4 |
% | Modulus | x % y | 5 |
** | Exponentiation | x ** y | 6 |
// | Floor division | x // y | 7 |
Python Assignment Operators
Assignment operators are used to assign values to variables:
Operator | Example | Same As | Num |
---|---|---|---|
= | x = 5 | x = 5 | 1 |
+= | x += 3 | x = x + 3 | 2 |
-= | x -= 3 | x = x – 3 | 3 |
*= | x *= 3 | x = x * 3 | 4 |
/= | x /= 3 | x = x / 3 | 5 |
%= | x %= 3 | x = x % 3 | 6 |
//= | x //= 3 | x = x // 3 | 7 |
**= | x **= 3 | x = x ** 3 | 8 |
&= | x &= 3 | x = x & 3 | 9 |
|= | x |= 3 | x = x | 3 | 10 |
^= | x ^= 3 | x = x ^ 3 | 11 |
>>= | x >>= 3 | x = x >> 3 | 12 |
<<= | x <<= 3 | x = x << 3 | 13 |
Python Comparison Operators
Comparison operators are used to compare two values:
Operator | Name | Example | Num |
---|---|---|---|
== | Equal | x == y | 1 |
!= | Not equal | x != y | 2 |
> | Greater than | x > y | 3 |
< | Less than | x < y | 4 |
>= | Greater than or equal to | x >= y | 5 |
<= | Less than or equal to | x <= y | 6 |
Python Logical Operators
Logical operators are used to combine conditional statements:
Operator | Description | Example | Num |
---|---|---|---|
and | Returns True if both statements are true | x < 5 and x < 10 | 1 |
or | Returns True if one of the statements is true | x < 5 or x < 4 | 2 |
not | Reverse the result, returns False if the result is true | not(x < 5 and x < 10) | 3 |
Python Identity Operators
Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location:
Operator | Description | Example | Num |
---|---|---|---|
is | Returns True if both variables are the same object | x is y | 1 |
is not | Returns True if both variables are not the same object | x is not y | 2 |
Python Membership Operators
Membership operators are used to test if a sequence is presented in an object:
Operator | Description | Example | Num |
---|---|---|---|
in | Returns True if a sequence with the specified value is present in the object | x in y | 1 |
not in | Returns True if a sequence with the specified value is not present in the object | x not in y | 2 |
Python Bitwise Operators
Bitwise operators are used to compare (binary) numbers:
Operator | Name | Description | Example | Num |
---|---|---|---|---|
& | AND | Sets each bit to 1 if both bits are 1 | x & y | 1 |
| | OR | Sets each bit to 1 if one of two bits is 1 | x | y | 2 |
^ | XOR | Sets each bit to 1 if only one of two bits is 1 | x ^ y | 3 |
~ | NOT | Inverts all the bits | ~x | 4 |
<< | Zero fill left shift | Shift left by pushing zeros in from the right and let the leftmost bits fall off | x << 2 | 5 |
>> | Signed right shift | Shift right by pushing copies of the leftmost bit in from the left, and let the rightmost bits fall off | x >> 2 | 6 |
Operator Precedence
Operator precedence describes the order in which operations are performed.
Example
Parentheses has the highest precedence, meaning that expressions inside parentheses must be evaluated first:
print((6 + 3) – (6 + 3))
Example
Multiplication *
has higher precedence than addition +
, and therefor multiplications are evaluated before additions:
print(100 + 5 * 3)
The precedence order is described in the table below, starting with the highest precedence at the top:
Operator | Description | Num |
---|---|---|
() |
Parentheses | 1 |
** |
Exponentiation | 2 |
+x -x ~x |
Unary plus, unary minus, and bitwise NOT | 3 |
* / // % |
Multiplication, division, floor division, and modulus | 4 |
+ - |
Addition and subtraction | 5 |
<< >> |
Bitwise left and right shifts | 6 |
& |
Bitwise AND | 7 |
^ |
Bitwise XOR | 8 |
| |
Bitwise OR | 9 |
== != > >= < <= is is not in not in |
Comparisons, identity, and membership operators | 10 |
not |
Logical NOT | 11 |
and |
AND | 12 |
or |
OR | 13 |
If two operators have the same precedence, the expression is evaluated from left to right.
Example
Addition +
and subtraction -
has the same precedence, and therefor we evaluate the expression from left to right:
print(5 + 4 – 7 + 3)
Python Lists
mylist = [“apple”, “banana”, “cherry”]
List
Lists are used to store multiple items in a single variable.
Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.
Lists are created using square brackets:
Example
Create a List:
thislist = [“apple”, “banana”, “cherry”]
print(thislist)
List Items
List items are ordered, changeable, and allow duplicate values.
List items are indexed, the first item has index [0]
, the second item has index [1]
etc.
Ordered
When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
If you add new items to a list, the new items will be placed at the end of the list.
Changeable
The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.
Allow Duplicates
Since lists are indexed, lists can have items with the same value:
Example
Lists allow duplicate values:
thislist = [“apple”, “banana”, “cherry”, “apple”, “cherry”]
print(thislist)
List Length
To determine how many items a list has, use the len()
function:
Example
Print the number of items in the list:
thislist = [“apple”, “banana”, “cherry”]
print(len(thislist))
List Items – Data Types
List items can be of any data type:
Example
String, int and boolean data types:
list1 = [“apple”, “banana”, “cherry”]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
A list can contain different data types:
Example
A list with strings, integers and boolean values:
list1 = [“abc”, 34, True, 40, “male”]
type()
From Python’s perspective, lists are defined as objects with the data type ‘list’:
<class ‘list’>
Example
What is the data type of a list?
mylist = [“apple”, “banana”, “cherry”]
print(type(mylist))
The list() Constructor
It is also possible to use the list() constructor when creating a new list.
Example
Using the list()
constructor to make a List:
thislist = list((“apple”, “banana”, “cherry”)) # note the double round-brackets
print(thislist)
Python – Access List Items
Access Items
List items are indexed and you can access them by referring to the index number:
Example
Print the second item of the list:
thislist = [“apple”, “banana”, “cherry”]
print(thislist[1])
Note: The first item has index 0.
Negative Indexing
Negative indexing means start from the end
-1
refers to the last item, -2
refers to the second last item etc.
Example
Print the last item of the list:
thislist = [“apple”, “banana”, “cherry”]
print(thislist[-1])
Range of Indexes
You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new list with the specified items.
Example
Return the third, fourth, and fifth item:
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”]
print(thislist[2:5])
Note: The search will start at index 2 (included) and end at index 5 (not included).
Example
This example returns the items from the beginning to, but NOT including, “kiwi”:
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”]
print(thislist[:4])
By leaving out the end value, the range will go on to the end of the list:
Example
This example returns the items from “cherry” to the end:
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”]
print(thislist[2:])
Range of Negative Indexes
Specify negative indexes if you want to start the search from the end of the list:
Example
This example returns the items from “orange” (-4) to, but NOT including “mango” (-1):
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”]
print(thislist[-4:-1])
Check if Item Exists
To determine if a specified item is present in a list use the in
keyword:
Example
Check if “apple” is present in the list:
thislist = [“apple”, “banana”, “cherry”]
if “apple” in thislist:
print(“Yes, ‘apple’ is in the fruits list”)
Python – Change List Items
Change Item Value
To change the value of a specific item, refer to the index number:
Example
Change the second item:
thislist = [“apple”, “banana”, “cherry”]
thislist[1] = “blackcurrant”
print(thislist)
Change a Range of Item Values
To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:
Example
Change the values “banana” and “cherry” with the values “blackcurrant” and “watermelon”:
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “mango”]
thislist[1:3] = [“blackcurrant”, “watermelon”]
print(thislist)
If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
Example
Change the second value by replacing it with two new values:
thislist = [“apple”, “banana”, “cherry”]
thislist[1:2] = [“blackcurrant”, “watermelon”]
print(thislist)
Note: The length of the list will change when the number of items inserted does not match the number of items replaced.
Example
Change the second and third value by replacing it with one value:
thislist = [“apple”, “banana”, “cherry”]
thislist[1:3] = [“watermelon”]
print(thislist)
Insert Items
To insert a new list item, without replacing any of the existing values, we can use the insert()
method.
The insert()
method inserts an item at the specified index:
Example
Insert “watermelon” as the third item:
thislist = [“apple”, “banana”, “cherry”]
thislist.insert(2, “watermelon”)
print(thislist)
Note: As a result of the example above, the list will now contain 4 items.
Python – Add List Items
Append Items
To add an item to the end of the list, use the append() method:
Example
Using the append()
method to append an item:
thislist = [“apple”, “banana”, “cherry”]
thislist.append(“orange”)
print(thislist)
Insert Items
To insert a list item at a specified index, use the insert()
method.
The insert()
method inserts an item at the specified index:
Example
Insert an item as the second position:
thislist = [“apple”, “banana”, “cherry”]
thislist.insert(1, “orange”)
print(thislist)
Note: As a result of the examples above, the lists will now contain 4 items.
Extend List
To append elements from another list to the current list, use the extend()
method.
Example
Add the elements of tropical
to thislist
:
thislist = [“apple”, “banana”, “cherry”]
tropical = [“mango”, “pineapple”, “papaya”]
thislist.extend(tropical)
print(thislist)
The elements will be added to the end of the list.
Add Any Iterable
The extend()
method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).
Example
Add elements of a tuple to a list:
thislist = [“apple”, “banana”, “cherry”]
thistuple = (“kiwi”, “orange”)
thislist.extend(thistuple)
print(thislist)
Python – Remove List Items
Remove Specified Item
The remove()
method removes the specified item.
Example
Remove “banana”:
thislist = [“apple”, “banana”, “cherry”]
thislist.remove(“banana”)
print(thislist)
Remove Specified Index
The pop()
method removes the specified index.
Example
Remove the second item:
thislist = [“apple”, “banana”, “cherry”]
thislist.pop(1)
print(thislist)
If you do not specify the index, the pop()
method removes the last item.
Example
Remove the last item:
thislist = [“apple”, “banana”, “cherry”]
thislist.pop()
print(thislist)
The del
keyword also removes the specified index:
Example
Remove the first item:
thislist = [“apple”, “banana”, “cherry”]
del thislist[0]
print(thislist)
The del
keyword can also delete the list completely.
Example
Delete the entire list:
thislist = [“apple”, “banana”, “cherry”]
del thislist
Clear the List
The clear()
method empties the list.
The list still remains, but it has no content.
Example
Clear the list content:
thislist = [“apple”, “banana”, “cherry”]
thislist.clear()
print(thislist)
Python – Loop Lists
Loop Through a List
You can loop through the list items by using a for
loop:
Example
Print all items in the list, one by one:
thislist = [“apple”, “banana”, “cherry”]
for x in thislist:
print(x)
Loop Through the Index Numbers
You can also loop through the list items by referring to their index number.
Use the range()
and len()
functions to create a suitable iterable.
Example
Print all items by referring to their index number:
thislist = [“apple”, “banana”, “cherry”]
for i in range(len(thislist)):
print(thislist[i])
The iterable created in the example above is [0, 1, 2]
.
Using a While Loop
You can loop through the list items by using a while
loop.
Use the len()
function to determine the length of the list, then start at 0 and loop your way through the list items by referring to their indexes.
Remember to increase the index by 1 after each iteration.
Example
Print all items, using a while
loop to go through all the index numbers
thislist = [“apple”, “banana”, “cherry”]
i = 0
while i < len(thislist):
print(thislist[i])
i = i + 1
Learn more about while
loops in our Python While Loops Chapter.
Looping Using List Comprehension
List Comprehension offers the shortest syntax for looping through lists:
Example
A short hand for
loop that will print all items in a list:
thislist = [“apple”, “banana”, “cherry”]
[print(x) for x in thislist]
Python – Copy Lists
Copy a List
You cannot copy a list simply by typing list2 = list1
, because: list2
will only be a reference to list1
, and changes made in list1
will automatically also be made in list2
.
There are ways to make a copy, one way is to use the built-in List method copy()
.
Example
Make a copy of a list with the copy()
method:
thislist = [“apple”, “banana”, “cherry”]
mylist = thislist.copy()
print(mylist)
Another way to make a copy is to use the built-in method list()
.
Example
Make a copy of a list with the list()
method:
thislist = [“apple”, “banana”, “cherry”]
mylist = list(thislist)
print(mylist)
Python – Join Lists
Join Two Lists
There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways are by using the +
operator.
Example
Join two list:
list1 = [“a”, “b”, “c”]
list2 = [1, 2, 3]list3 = list1 + list2
print(list3)
Another way to join two lists is by appending all the items from list2 into list1, one by one:
Example
Append list2 into list1:
list1 = [“a”, “b” , “c”]
list2 = [1, 2, 3]for x in list2:
list1.append(x)print(list1)
Or you can use the extend()
method, which purpose is to add elements from one list to another list:
Example
Use the extend()
method to add list2 at the end of list1:
list1 = [“a”, “b” , “c”]
list2 = [1, 2, 3]list1.extend(list2)
print(list1)
Python Tuples
mytuple = (“apple”, “banana”, “cherry”)
Tuple
Tuples are used to store multiple items in a single variable.
Tuple is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Set, and Dictionary, all with different qualities and usage.
A tuple is a collection which is ordered and unchangeable.
Tuples are written with round brackets.
Example
Create a Tuple:
thistuple = (“apple”, “banana”, “cherry”)
print(thistuple)
Tuple Items
Tuple items are ordered, unchangeable, and allow duplicate values.
Tuple items are indexed, the first item has index [0]
, the second item has index [1]
etc.
Ordered
When we say that tuples are ordered, it means that the items have a defined order, and that order will not change.
Unchangeable
Tuples are unchangeable, meaning that we cannot change, add or remove items after the tuple has been created.
Allow Duplicates
Since tuples are indexed, they can have items with the same value:
Example
Tuples allow duplicate values:
thistuple = (“apple”, “banana”, “cherry”, “apple”, “cherry”)
print(thistuple)
Tuple Length
To determine how many items a tuple has, use the len()
function:
Example
Print the number of items in the tuple:
thistuple = (“apple”, “banana”, “cherry”)
print(len(thistuple))
Create Tuple With One Item
To create a tuple with only one item, you have to add a comma after the item, otherwise Python will not recognize it as a tuple.
Example
One item tuple, remember the comma:
thistuple = (“apple”,)
print(type(thistuple))#NOT a tuple
thistuple = (“apple”)
print(type(thistuple))
Tuple Items – Data Types
Tuple items can be of any data type:
Example
String, int and boolean data types:
tuple1 = (“apple”, “banana”, “cherry”)
tuple2 = (1, 5, 7, 9, 3)
tuple3 = (True, False, False)
A tuple can contain different data types:
Example
A tuple with strings, integers and boolean values:
tuple1 = (“abc”, 34, True, 40, “male”)
type()
From Python’s perspective, tuples are defined as objects with the data type ‘tuple’:
<class ‘tuple’>
Example
What is the data type of a tuple?
mytuple = (“apple”, “banana”, “cherry”)
print(type(mytuple))
The tuple() Constructor
It is also possible to use the tuple() constructor to make a tuple.
Example
Using the tuple() method to make a tuple:
thistuple = tuple((“apple”, “banana”, “cherry”)) # note the double round-brackets
print(thistuple)
Python – Access Tuple Items
Access Tuple Items
You can access tuple items by referring to the index number, inside square brackets:
Example
Print the second item in the tuple:
thistuple = (“apple”, “banana”, “cherry”)
print(thistuple[1])
Note: The first item has index 0.
Negative Indexing
Negative indexing means start from the end.
-1
refers to the last item, -2
refers to the second last item etc.
Example
Print the last item of the tuple:
thistuple = (“apple”, “banana”, “cherry”)
print(thistuple[-1])
Range of Indexes
You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new tuple with the specified items.
Example
Return the third, fourth, and fifth item:
thistuple = (“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”)
print(thistuple[2:5])
Note: The search will start at index 2 (included) and end at index 5 (not included).
Remember that the first item has index 0.
Example
This example returns the items from the beginning to, but NOT included, “kiwi”:
thistuple = (“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”)
print(thistuple[:4])
By leaving out the end value, the range will go on to the end of the list:
Example
This example returns the items from “cherry” and to the end:
thistuple = (“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”)
print(thistuple[2:])
Range of Negative Indexes
Specify negative indexes if you want to start the search from the end of the tuple:
Example
This example returns the items from index -4 (included) to index -1 (excluded)
thistuple = (“apple”, “banana”, “cherry”, “orange”, “kiwi”, “melon”, “mango”)
print(thistuple[-4:-1])
Check if Item Exists
To determine if a specified item is present in a tuple use the in
keyword:
Example
Check if “apple” is present in the tuple:
thistuple = (“apple”, “banana”, “cherry”)
if “apple” in thistuple:
print(“Yes, ‘apple’ is in the fruits tuple”)
Python – Update Tuples
Tuples are unchangeable, meaning that you cannot change, add, or remove items once the tuple is created.
But there are some workarounds.
Change Tuple Values
Once a tuple is created, you cannot change its values. Tuples are unchangeable, or immutable as it also is called.
But there is a workaround. You can convert the tuple into a list, change the list, and convert the list back into a tuple.
Example
Convert the tuple into a list to be able to change it:
x = (“apple”, “banana”, “cherry”)
y = list(x)
y[1] = “kiwi”
x = tuple(y)print(x)
Add Items
Since tuples are immutable, they do not have a build-in append()
method, but there are other ways to add items to a tuple.
1. Convert into a list: Just like the workaround for changing a tuple, you can convert it into a list, add your item(s), and convert it back into a tuple.
Example
Convert the tuple into a list, add “orange”, and convert it back into a tuple:
thistuple = (“apple”, “banana”, “cherry”)
y = list(thistuple)
y.append(“orange”)
thistuple = tuple(y)
2. Add tuple to a tuple. You are allowed to add tuples to tuples, so if you want to add one item, (or many), create a new tuple with the item(s), and add it to the existing tuple:
Example
Create a new tuple with the value “orange”, and add that tuple:
thistuple = (“apple”, “banana”, “cherry”)
y = (“orange”,)
thistuple += yprint(thistuple)
Note: When creating a tuple with only one item, remember to include a comma after the item, otherwise it will not be identified as a tuple.
Remove Items
Note: You cannot remove items in a tuple.
Tuples are unchangeable, so you cannot remove items from it, but you can use the same workaround as we used for changing and adding tuple items:
Example
Convert the tuple into a list, remove “apple”, and convert it back into a tuple:
thistuple = (“apple”, “banana”, “cherry”)
y = list(thistuple)
y.remove(“apple”)
thistuple = tuple(y)
Or you can delete the tuple completely:
Example
The del
keyword can delete the tuple completely:
thistuple = (“apple”, “banana”, “cherry”)
del thistuple
print(thistuple)#this will raise an error because the tuple no longer exists
Python – Join Tuples
Join Two Tuples
To join two or more tuples you can use the +
operator:
Example
Join two tuples:
tuple1 = (“a”, “b” , “c”)
tuple2 = (1, 2, 3)tuple3 = tuple1 + tuple2
print(tuple3)
Multiply Tuples
If you want to multiply the content of a tuple a given number of times, you can use the *
operator:
Example
Multiply the fruits tuple by 2:
fruits = (“apple”, “banana”, “cherry”)
mytuple = fruits * 2print(mytuple)
Python Sets
myset = {“apple”, “banana”, “cherry”}
Set
Sets are used to store multiple items in a single variable.
Set is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Tuple, and Dictionary, all with different qualities and usage.
A set is a collection which is unordered, unchangeable*, and unindexed.
* Note: Set items are unchangeable, but you can remove items and add new items.
Sets are written with curly brackets.
Example
Create a Set:
thisset = {“apple”, “banana”, “cherry”}
print(thisset)
Note: Sets are unordered, so you cannot be sure in which order the items will appear.
Set Items
Set items are unordered, unchangeable, and do not allow duplicate values.
Unordered
Unordered means that the items in a set do not have a defined order.
Set items can appear in a different order every time you use them, and cannot be referred to by index or key.
Unchangeable
Set items are unchangeable, meaning that we cannot change the items after the set has been created.
Once a set is created, you cannot change its items, but you can remove items and add new items.
Duplicates Not Allowed
Sets cannot have two items with the same value.
Example
Duplicate values will be ignored:
thisset = {“apple”, “banana”, “cherry”, “apple”}
print(thisset)
Note: The values
True
and1
are considered the same value in sets, and are treated as duplicates:
Example
True
and 1
is considered the same value:
thisset = {“apple”, “banana”, “cherry”, True, 1, 2}
print(thisset)
Get the Length of a Set
To determine how many items a set has, use the len()
function.
Example
Get the number of items in a set:
thisset = {“apple”, “banana”, “cherry”}
print(len(thisset))
Set Items – Data Types
Set items can be of any data type:
Example
String, int and boolean data types:
set1 = {“apple”, “banana”, “cherry”}
set2 = {1, 5, 7, 9, 3}
set3 = {True, False, False}
A set can contain different data types:
Example
A set with strings, integers and boolean values:
set1 = {“abc”, 34, True, 40, “male”}
type()
From Python’s perspective, sets are defined as objects with the data type ‘set’:
<class ‘set’>
Example
What is the data type of a set?
myset = {“apple”, “banana”, “cherry”}
print(type(myset))
The set() Constructor
It is also possible to use the set() constructor to make a set.
Example
Using the set() constructor to make a set:
thisset = set((“apple”, “banana”, “cherry”)) # note the double round-brackets
print(thisset)
Python – Access Set Items
Access Items
You cannot access items in a set by referring to an index or a key.
But you can loop through the set items using a for
loop, or ask if a specified value is present in a set, by using the in
keyword.
Example
Loop through the set, and print the values:
thisset = {“apple”, “banana”, “cherry”}
for x in thisset:
print(x)
Example
Check if “banana” is present in the set:
thisset = {“apple”, “banana”, “cherry”}
print(“banana” in thisset)
Change Items
Once a set is created, you cannot change its items, but you can add new items.
Python – Add Set Items
Add Items
Once a set is created, you cannot change its items, but you can add new items.
To add one item to a set use the add()
method.
Example
Add an item to a set, using the add()
method:
thisset = {“apple”, “banana”, “cherry”}
thisset.add(“orange”)
print(thisset)
Add Sets
To add items from another set into the current set, use the update()
method.
Example
Add elements from tropical
into thisset
:
thisset = {“apple”, “banana”, “cherry”}
tropical = {“pineapple”, “mango”, “papaya”}thisset.update(tropical)print(thisset)
Add Any Iterable
The object in the update()
method does not have to be a set, it can be any iterable object (tuples, lists, dictionaries etc.).
Example
Add elements of a list to at set:
thisset = {“apple”, “banana”, “cherry”}
mylist = [“kiwi”, “orange”]thisset.update(mylist)print(thisset)
Python – Remove Set Items
Remove Item
To remove an item in a set, use the remove()
, or the discard()
method.
Example
Remove “banana” by using the remove()
method:
thisset = {“apple”, “banana”, “cherry”}
thisset.remove(“banana”)
print(thisset)
Note: If the item to remove does not exist,
remove()
will raise an error.
Example
Remove “banana” by using the discard()
method:
thisset = {“apple”, “banana”, “cherry”}
thisset.discard(“banana”)
print(thisset)
Note: If the item to remove does not exist,
discard()
will NOT raise an error.
You can also use the
pop()
method to remove an item, but this method will remove a random item, so you cannot be sure what item that gets removed.The return value of the
pop()
method is the removed item.
Example
Remove a random item by using the pop()
method:
thisset = {“apple”, “banana”, “cherry”}
x =thisset.pop()
print(x)
print(thisset)
Note: Sets are unordered, so when using the
pop()
method, you do not know which item that gets removed.
Example
The clear()
method empties the set:
thisset = {“apple”, “banana”, “cherry”}
thisset.clear()
print(thisset)
Example
The del
keyword will delete the set completely:
thisset = {“apple”, “banana”, “cherry”}
delthisset
print(thisset)
Python – Join Sets
Join Two Sets
There are several ways to join two or more sets in Python.
You can use the union()
method that returns a new set containing all items from both sets, or the update()
method that inserts all the items from one set into another:
Example
The union()
method returns a new set with all items from both sets:
set1 = {“a”, “b” , “c”}
set2 = {1, 2, 3}set3 = set1.union(set2)
print(set3)
Example
The update()
method inserts the items in set2 into set1:
set1 = {“a”, “b” , “c”}
set2 = {1, 2, 3}set1.update(set2)
print(set1)
Note: Both
union()
andupdate()
will exclude any duplicate items.
Keep ONLY the Duplicates
The intersection_update()
method will keep only the items that are present in both sets.
Example
Keep the items that exist in both set x
, and set y
:
x = {“apple”, “banana”, “cherry”}
y = {“google”, “microsoft”, “apple”}x.intersection_update(y)print(x)
The intersection()
method will return a new set, that only contains the items that are present in both sets.
Example
Return a set that contains the items that exist in both set x
, and set y
:
x = {“apple”, “banana”, “cherry”}
y = {“google”, “microsoft”, “apple”}z = x.intersection(y)print(z)
Keep All, But NOT the Duplicates
The symmetric_difference_update()
method will keep only the elements that are NOT present in both sets.
Example
Keep the items that are not present in both sets:
x = {“apple”, “banana”, “cherry”}
y = {“google”, “microsoft”, “apple”}x.symmetric_difference_update(y)print(x)
The symmetric_difference()
method will return a new set, that contains only the elements that are NOT present in both sets.
Example
Return a set that contains all items from both sets, except items that are present in both:
x = {“apple”, “banana”, “cherry”}
y = {“google”, “microsoft”, “apple”}z = x.symmetric_difference(y)print(z)
Note: The values
True
and1
are considered the same value in sets, and are treated as duplicates:
Example
True
and 1
is considered the same value:
x = {“apple”, “banana”, “cherry”, True}
y = {“google”, 1, “apple”, 2}z = x.symmetric_difference(y)
Python Dictionaries
thisdict = {
“brand”: “Ford”,
“model”: “Mustang”,
“year”: 1964
}
Dictionary
Dictionaries are used to store data values in key:value pairs.
A dictionary is a collection which is ordered*, changeable and do not allow duplicates.
As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
Dictionaries are written with curly brackets, and have keys and values:
Example
Create and print a dictionary:
thisdict = {
“brand”: “Ford”,
“model”: “Mustang”,
“year”: 1964
}
print(thisdict)
Dictionary Items
Dictionary items are ordered, changeable, and does not allow duplicates.
Dictionary items are presented in key:value pairs, and can be referred to by using the key name.
Example
Print the “brand” value of the dictionary:
thisdict = {
“brand”: “Ford”,
“model”: “Mustang”,
“year”: 1964
}
print(thisdict[“brand”])
Ordered or Unordered?
As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.
Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.
Changeable
Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.
Duplicates Not Allowed
Dictionaries cannot have two items with the same key:
Example
Duplicate values will overwrite existing values:
thisdict = {
“brand”: “Ford”,
“model”: “Mustang”,
“year”: 1964,
“year”: 2020
}
print(thisdict)
Dictionary Length
To determine how many items a dictionary has, use the len()
function:
Example
Print the number of items in the dictionary:
print(len(thisdict))
Dictionary Items – Data Types
The values in dictionary items can be of any data type:
Example
String, int, boolean, and list data types:
thisdict = {
“brand”: “Ford”,
“electric”: False,
“year”: 1964,
“colors”: [“red”, “white”, “blue”]
}
type()
From Python’s perspective, dictionaries are defined as objects with the data type ‘dict’:
<class ‘dict’>
Example
Print the data type of a dictionary:
thisdict = {
“brand”: “Ford”,
“model”: “Mustang”,
“year”: 1964
}
print(type(thisdict))
The dict() Constructor
It is also possible to use the dict() constructor to make a dictionary.
Example
Using the dict() method to make a dictionary:
thisdict = dict(name = “John”, age = 36, country = “Norway”)
print(thisdict)
Python If … Else
Python Conditions and If statements
Python supports the usual logical conditions from mathematics:
- Equals: a == b
- Not Equals: a != b
- Less than: a < b
- Less than or equal to: a <= b
- Greater than: a > b
- Greater than or equal to: a >= b
These conditions can be used in several ways, most commonly in “if statements” and loops.
An “if statement” is written by using the if keyword.
Example
If statement:
a = 33
b = 200
if b > a:
print(“b is greater than a”)
In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that “b is greater than a”.
Indentation
Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.
Example
If statement, without indentation (will raise an error):
a = 33
b = 200
if b > a:
print(“b is greater than a”) # you will get an error
Elif
The elif keyword is Python’s way of saying “if the previous conditions were not true, then try this condition”.
Example
a = 33
b = 33
if b > a:
print(“b is greater than a”)
elif a == b:
print(“a and b are equal”)
In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that “a and b are equal”.
Else
The else keyword catches anything which isn’t caught by the preceding conditions.
Example
a = 200
b = 33
if b > a:
print(“b is greater than a”)
elif a == b:
print(“a and b are equal”)
else:
print(“a is greater than b”)
In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that “a is greater than b”.
You can also have an else
without the elif
:
Example
a = 200
b = 33
if b > a:
print(“b is greater than a”)
else:
print(“b is not greater than a”)
Short Hand If
If you have only one statement to execute, you can put it on the same line as the if statement.
Example
One line if statement:
if a > b: print(“a is greater than b”)
Short Hand If … Else
If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:
Example
One line if else statement:
a = 2
b = 330
print(“A”) if a > b else print(“B”)
This technique is known as Ternary Operators, or Conditional Expressions.
You can also have multiple else statements on the same line:
Example
One line if else statement, with 3 conditions:
a = 330
b = 330
print(“A”) if a > b else print(“=”) if a == b else print(“B”)
And
The and keyword is a logical operator, and is used to combine conditional statements:
Example
Test if
a
is greater thanb
, AND ifc
is greater thana
:a = 200
b = 33
c = 500
if a > b and c > a:
print(“Both conditions are True”)
Or
The or
keyword is a logical operator, and is used to combine conditional statements:
Example
Test if
a
is greater thanb
, OR ifa
is greater thanc
:a = 200
b = 33
c = 500
if a > b or a > c:
print(“At least one of the conditions is True”)
Not
The not
keyword is a logical operator, and is used to reverse the result of the conditional statement:
Example
Test if
a
is NOT greater thanb
:a = 33
b = 200
if not a > b:
print(“a is NOT greater than b”)
Nested If
You can have if
statements inside if
statements, this is called nested if
statements.
Example
x = 41
if x > 10:
print(“Above ten,”)
if x > 20:
print(“and also above 20!”)
else:
print(“but not above 20.”)
The pass Statement
if
statements cannot be empty, but if you for some reason have an if
statement with no content, put in the pass
statement to avoid getting an error.
Example
a = 33
b = 200if b > a:
pass
Python While Loops
Python Loops
Python has two primitive loop commands:
- while loops
- for loops
The while Loop
With the while loop we can execute a set of statements as long as a condition is true.
Example
Print i as long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
Note: remember to increment i, or else the loop will continue forever.
The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.
The break Statement
With the break statement we can stop the loop even if the while condition is true:
Example
Exit the loop when i is 3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
The continue Statement
With the continue statement we can stop the current iteration, and continue with the next:
Example
Continue to the next iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
The else Statement
With the else statement we can run a block of code once when the condition no longer is true:
Example
Print a message once the condition is false:
i = 1
while i < 6:
print(i)
i += 1
else:
print(“i is no longer less than 6”)
Python For Loops
Python For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
Example
Print each fruit in a fruit list:
fruits = [“apple”, “banana”, “cherry”]
for x in fruits:
print(x)
The for loop does not require an indexing variable to set beforehand.
Looping Through a String
Even strings are iterable objects, they contain a sequence of characters:
Example
Loop through the letters in the word “banana”:
for x in “banana”:
print(x)
The break Statement
With the break statement we can stop the loop before it has looped through all the items:
Example
Exit the loop when
x
is “banana”:fruits = [“apple”, “banana”, “cherry”]
for x in fruits:
print(x)
if x == “banana”:
break
Example
Exit the loop when
x
is “banana”, but this time the break comes before the print:fruits = [“apple”, “banana”, “cherry”]
for x in fruits:
if x == “banana”:
break
print(x)
The continue Statement
With the continue statement we can stop the current iteration of the loop, and continue with the next:
Example
Do not print banana:
fruits = [“apple”, “banana”, “cherry”]
for x in fruits:
if x == “banana”:
continue
print(x)
The range() Function
To loop through a set of code a specified number of times, we can use the range() function,
The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.
Example
Using the range() function:
for x in range(6):
print(x)
Note that range(6) is not the values of 0 to 6, but the values 0 to 5.
The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):
Example
Using the start parameter:
for x in range(2, 6):
print(x)
The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):
Example
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3):
print(x)
Else in For Loop
The else
keyword in a for
loop specifies a block of code to be executed when the loop is finished:
Example
Print all numbers from 0 to 5, and print a message when the loop has ended:
for x in range(6):
print(x)
else:
print(“Finally finished!”)
Note: The
else
block will NOT be executed if the loop is stopped by abreak
statement.
Example
Break the loop when
x
is 3, and see what happens with theelse
block:for x in range(6):
if x == 3: break
print(x)
else:
print(“Finally finished!”)
Nested Loops
A nested loop is a loop inside a loop.
The “inner loop” will be executed one time for each iteration of the “outer loop”:
Example
Print each adjective for every fruit:
adj = [“red”, “big”, “tasty”]
fruits = [“apple”, “banana”, “cherry”]for x in adj:
for y in fruits:
print(x, y)
The pass Statement
for
loops cannot be empty, but if you for some reason have a for
loop with no content, put in the pass
statement to avoid getting an error.
Example
for x in [0, 1, 2]:
pass
Python Functions
A function is a block of code which only runs when it is called.
You can pass data, known as parameters, into a function.
A function can return data as a result.
Creating a Function
In Python a function is defined using the def keyword:
Example
def my_function():
print(“Hello from a function”)
Calling a Function
To call a function, use the function name followed by parenthesis:
Example
def my_function():
print(“Hello from a function”)my_function()
Arguments
Information can be passed into functions as arguments.
Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.
The following example has a function with one argument (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:
Example
def my_function(fname):
print(fname + ” Refsnes”)my_function(“Emil”)
my_function(“Tobias”)
my_function(“Linus”)
Arguments are often shortened to args in Python documentations.
Parameters or Arguments?
The terms parameter and argument can be used for the same thing: information that are passed into a function.
From a function’s perspective:
A parameter is the variable listed inside the parentheses in the function definition.
An argument is the value that is sent to the function when it is called.
Number of Arguments
By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.
Example
This function expects 2 arguments, and gets 2 arguments:
def my_function(fname, lname):
print(fname + ” “ + lname)my_function(“Emil”, “Refsnes”)
If you try to call the function with 1 or 3 arguments, you will get an error:
Example
This function expects 2 arguments, but gets only 1:
def my_function(fname, lname):
print(fname + ” “ + lname)my_function(“Emil”)
Arbitrary Arguments, *args
If you do not know how many arguments that will be passed into your function, add a *
before the parameter name in the function definition.
This way the function will receive a tuple of arguments, and can access the items accordingly:
Example
If the number of arguments is unknown, add a *
before the parameter name:
def my_function(*kids):
print(“The youngest child is “ + kids[2])my_function(“Emil”, “Tobias”, “Linus”)
Arbitrary Arguments are often shortened to *args in Python documentations.
Keyword Arguments
You can also send arguments with the key = value syntax.
This way the order of the arguments does not matter.
Example
def my_function(child3, child2, child1):
print(“The youngest child is “ + child3)my_function(child1 = “Emil”, child2 = “Tobias”, child3 = “Linus”)
The phrase Keyword Arguments are often shortened to kwargs in Python documentations.
Arbitrary Keyword Arguments, **kwargs
If you do not know how many keyword arguments that will be passed into your function, add two asterisk: **
before the parameter name in the function definition.
This way the function will receive a dictionary of arguments, and can access the items accordingly:
Example
If the number of keyword arguments is unknown, add a double **
before the parameter name:
def my_function(**kid):
print(“His last name is “ + kid[“lname”])my_function(fname = “Tobias”, lname = “Refsnes”)
Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.
Default Parameter Value
The following example shows how to use a default parameter value.
If we call the function without argument, it uses the default value:
Example
def my_function(country = “Norway”):
print(“I am from “ + country)my_function(“Sweden”)
my_function(“India”)
my_function()
my_function(“Brazil”)
Passing a List as an Argument
You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.
E.g. if you send a List as an argument, it will still be a List when it reaches the function:
Example
def my_function(food):
for x in food:
print(x)fruits = [“apple”, “banana”, “cherry”]my_function(fruits)
Return Values
To let a function return a value, use the return
statement:
Example
def my_function(x):
return 5 * x
print(my_function(3))
print(my_function(5))
print(my_function(9))
The pass Statement
function
definitions cannot be empty, but if you for some reason have a function
definition with no content, put in the pass
statement to avoid getting an error.
Example
def myfunction():
pass
Recursion
Python also accepts function recursion, which means a defined function can call itself.
Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.
The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.
In this example, tri_recursion() is a function that we have defined to call itself (“recurse”). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).
To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.
Example
Recursion Example
def tri_recursion(k):
if(k > 0):
result = k + tri_recursion(k – 1)
print(result)
else:
result = 0
return resultprint(“\n\nRecursion Example Results”)
tri_recursion(6)
Python Lambda
A lambda function is a small anonymous function.
A lambda function can take any number of arguments, but can only have one expression.
Syntax
lambda arguments : expression
The expression is executed and the result is returned:
Example
Add 10 to argument a
, and return the result:
x = lambda a : a + 10
print(x(5))
Lambda functions can take any number of arguments:
Example
Multiply argument a
with argument b
and return the result:
x = lambda a, b : a * b
print(x(5, 6))
Example
Summarize argument a
, b
, and c
and return the result:
x = lambda a, b, c : a + b + c
print(x(5, 6, 2))
Why Use Lambda Functions?
The power of lambda is better shown when you use them as an anonymous function inside another function.
Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:
def myfunc(n):
return lambda a : a * n
Use that function definition to make a function that always doubles the number you send in:
Example
def myfunc(n):
return lambda a : a * nmydoubler = myfunc(2)print(mydoubler(11))
Or, use the same function definition to make a function that always triples the number you send in:
Example
def myfunc(n):
return lambda a : a * nmytripler = myfunc(3)print(mytripler(11))
Or, use the same function definition to make both functions, in the same program:
Example
def myfunc(n):
return lambda a : a * nmydoubler = myfunc(2)
mytripler = myfunc(3)print(mydoubler(11))
print(mytripler(11))
Use lambda functions when an anonymous function is required for a short period of time.
Python Arrays
Arrays
Note: This page shows you how to use LISTS as ARRAYS, however, to work with arrays in Python you will have to import a library, like the NumPy library.
Example
Create an array containing car names:
cars = [“Ford”, “Volvo”, “BMW”]
What is an Array?
An array is a special variable, which can hold more than one value at a time.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
car1 = “Ford”
car2 = “Volvo”
car3 = “BMW”
However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can access the values by referring to an index number.
Access the Elements of an Array
You refer to an array element by referring to the index number.
Example
Get the value of the first array item:
x = cars[0]
Example
Modify the value of the first array item:
cars[0] = “Toyota”
The Length of an Array
Use the len()
method to return the length of an array (the number of elements in an array).
Example
Return the number of elements in the cars
array:
x = len(cars)
Note: The length of an array is always one more than the highest array index.
Looping Array Elements
You can use the for in
loop to loop through all the elements of an array.
Example
Print each item in the cars
array:
for x in cars:
print(x)
Adding Array Elements
You can use the append()
method to add an element to an array.
Example
Add one more element to the cars
array:
cars.append(“Honda”)
Removing Array Elements
You can use the pop()
method to remove an element from the array.
Example
Delete the second element of the cars
array:
cars.pop(1)
You can also use the remove()
method to remove an element from the array.
Example
Delete the element that has the value “Volvo”:
cars.remove(“Volvo”)
Note: The list’s
remove()
method only removes the first occurrence of the specified value.
Array Methods
Python has a set of built-in methods that you can use on lists/arrays.
Method | Description |
---|---|
append() | Adds an element at the end of the list |
clear() | Removes all the elements from the list |
copy() | Returns a copy of the list |
count() | Returns the number of elements with the specified value |
extend() | Add the elements of a list (or any iterable), to the end of the current list |
index() | Returns the index of the first element with the specified value |
insert() | Adds an element at the specified position |
pop() | Removes the element at the specified position |
remove() | Removes the first item with the specified value |
reverse() | Reverses the order of the list |
sort() | Sorts the list |
Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.
Python Classes and Objects
Python Classes/Objects
Python is an object oriented programming language.
Almost everything in Python is an object, with its properties and methods.
A Class is like an object constructor, or a “blueprint” for creating objects.
Create a Class
To create a class, use the keyword class
:
Example
Create a class named MyClass, with a property named x:
class MyClass:
x = 5
Create Object
Now we can use the class named MyClass to create objects:
Example
Create an object named p1, and print the value of x:
p1 = MyClass()
print(p1.x)
The __init__() Function
The examples above are classes and objects in their simplest form, and are not really useful in real life applications.
To understand the meaning of classes we have to understand the built-in __init__() function.
All classes have a function called __init__(), which is always executed when the class is being initiated.
Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:
Example
Create a class named Person, use the __init__() function to assign values for name and age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = agep1 = Person(“John”, 36)print(p1.name)
print(p1.age)
Note: The
__init__()
function is called automatically every time the class is being used to create a new object.
The __str__() Function
The __str__() function controls what should be returned when the class object is represented as a string.
If the __str__() function is not set, the string representation of the object is returned:
Example
The string representation of an object WITHOUT the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = agep1 = Person(“John”, 36)print(p1)
Example
The string representation of an object WITH the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age def __str__(self):
return f“{self.name}({self.age})”p1 = Person(“John”, 36)print(p1)
Object Methods
Objects can also contain methods. Methods in objects are functions that belong to the object.
Let us create a method in the Person class:
Example
Insert a function that prints a greeting, and execute it on the p1 object:
class Person:
def __init__(self, name, age):
self.name = name
self.age = agedef myfunc(self):
print(“Hello my name is “ + self.name)p1 = Person(“John”, 36)
p1.myfunc()
Note: The
self
parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.
The self Parameter
The self
parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.
It does not have to be named self
, you can call it whatever you like, but it has to be the first parameter of any function in the class:
Example
Use the words mysillyobject and abc instead of self:
class Person:
def __init__(mysillyobject, name, age):
mysillyobject.name = name
mysillyobject.age = agedef myfunc(abc):
print(“Hello my name is “ + abc.name)p1 = Person(“John”, 36)
p1.myfunc()
Modify Object Properties
You can modify properties on objects like this:
Example
Set the age of p1 to 40:
p1.age = 40
Delete Object Properties
You can delete properties on objects by using the del
keyword:
Example
Delete the age property from the p1 object:
del p1.age
Delete Objects
You can delete objects by using the del
keyword:
Example
Delete the p1 object:
del p1
The pass Statement
class
definitions cannot be empty, but if you for some reason have a class
definition with no content, put in the pass
statement to avoid getting an error.
Example
class Person:
pass
Python Inheritance
Python Inheritance
Inheritance allows us to define a class that inherits all the methods and properties from another class.
Parent class is the class being inherited from, also called base class.
Child class is the class that inherits from another class, also called derived class.
Create a Parent Class
Any class can be a parent class, so the syntax is the same as creating any other class:
Example
Create a class named Person
, with firstname
and lastname
properties, and a printname
method:
class Person:
def __init__(self, fname, lname):
self.firstname = fname
self.lastname = lnamedef printname(self):
print(self.firstname, self.lastname)#Use the Person class to create an object, and then execute the printname method:
x = Person(“John”, “Doe”)
x.printname()
Create a Child Class
To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:
Example
Create a class named Student
, which will inherit the properties and methods from the Person
class:
class Student(Person):
pass
Note: Use the
pass
keyword when you do not want to add any other properties or methods to the class.
Now the Student class has the same properties and methods as the Person class.
Example
Use the Student
class to create an object, and then execute the printname
method:
x = Student(“Mike”, “Olsen”)
x.printname()
Add the __init__() Function
So far we have created a child class that inherits the properties and methods from its parent.
We want to add the __init__()
function to the child class (instead of the pass
keyword).
Note: The
__init__()
function is called automatically every time the class is being used to create a new object.
Example
Add the __init__()
function to the Student
class:
class Student(Person):
def __init__(self, fname, lname):
#add properties etc.
When you add the
__init__()
function, the child class will no longer inherit the parent’s__init__()
function.
Note: The child’s
__init__()
function overrides the inheritance of the parent’s__init__()
function.
To keep the inheritance of the parent’s
__init__()
function, add a call to the parent’s__init__()
function:
Example
class Student(Person):
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__()
function.
Use the super() Function
Python also has a super()
function that will make the child class inherit all the methods and properties from its parent:
Example
class Student(Person):
def __init__(self, fname, lname):
super().__init__(fname, lname)
By using the super()
function, you do not have to use the name of the parent element, it will automatically inherit the methods and properties from its parent.
Add Properties
Example
Add a property called graduationyear
to the Student
class:
class Student(Person):
def __init__(self, fname, lname):
super().__init__(fname, lname)
self.graduationyear = 2019
In the example below, the year 2019
should be a variable, and passed into the Student
class when creating student objects. To do so, add another parameter in the __init__() function:
Example
Add a year
parameter, and pass the correct year when creating objects:
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = yearx = Student(“Mike”, “Olsen”, 2019)
Add Methods
Example
Add a method called welcome
to the Student
class:
class Student(Person):
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = yeardef welcome(self):
print(“Welcome”, self.firstname, self.lastname, “to the class of”, self.graduationyear)
If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.
Python Iterators
Python Iterators
An iterator is an object that contains a countable number of values.
An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.
Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__()
and __next__()
.
Iterator vs Iterable
Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.
All these objects have a iter()
method which is used to get an iterator:
Example
Return an iterator from a tuple, and print each value:
mytuple = (“apple”, “banana”, “cherry”)
myit = iter(mytuple)print(next(myit))
print(next(myit))
print(next(myit))
Even strings are iterable objects, and can return an iterator:
Example
Strings are also iterable objects, containing a sequence of characters:
mystr = “banana”
myit = iter(mystr)print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
Looping Through an Iterator
We can also use a for
loop to iterate through an iterable object:
Example
Iterate the values of a tuple:
mytuple = (“apple”, “banana”, “cherry”)
for x in mytuple:
print(x)
Example
Iterate the characters of a string:
mystr = “banana”
for x in mystr:
print(x)
The for
loop actually creates an iterator object and executes the next() method for each loop.
Create an Iterator
To create an object/class as an iterator you have to implement the methods __iter__()
and __next__()
to your object.
As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__()
, which allows you to do some initializing when the object is being created.
The __iter__()
method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.
The __next__()
method also allows you to do operations, and must return the next item in the sequence.
Example
Create an iterator that returns numbers, starting with 1, and each sequence will increase by one (returning 1,2,3,4,5 etc.):
class MyNumbers:
def __iter__(self):
self.a = 1
return selfdef __next__(self):
x = self.a
self.a += 1
return xmyclass = MyNumbers()
myiter = iter(myclass)print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
StopIteration
The example above would continue forever if you had enough next() statements, or if it was used in a for
loop.
To prevent the iteration to go on forever, we can use the StopIteration
statement.
In the __next__()
method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:
Example
Stop after 20 iterations:
class MyNumbers:
def __iter__(self):
self.a = 1
return selfdef __next__(self):
if self.a <= 20:
x = self.a
self.a += 1
return x
else:
raise StopIterationmyclass = MyNumbers()
myiter =iter(myclass)for x in myiter:
print(x)
Python Scope
A variable is only available from inside the region it is created. This is called scope.
Local Scope
A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.
Example
A variable created inside a function is available inside that function:
def myfunc():
x = 300
print(x)myfunc()
Function Inside Function
As explained in the example above, the variable x
is not available outside the function, but it is available for any function inside the function:
Example
The local variable can be accessed from a function within the function:
def myfunc():
x = 300
def myinnerfunc():
print(x)
myinnerfunc()myfunc()
Global Scope
A variable created in the main body of the Python code is a global variable and belongs to the global scope.
Global variables are available from within any scope, global and local.
Example
A variable created outside of a function is global and can be used by anyone:
x = 300
def myfunc():
print(x)myfunc()
print(x)
Naming Variables
If you operate with the same variable name inside and outside of a function, Python will treat them as two separate variables, one available in the global scope (outside the function) and one available in the local scope (inside the function):
Example
The function will print the local x
, and then the code will print the global x
:
x = 300
def myfunc():
x = 200
print(x)myfunc()
print(x)
Global Keyword
If you need to create a global variable, but are stuck in the local scope, you can use the global
keyword.
The global
keyword makes the variable global.
Example
If you use the global
keyword, the variable belongs to the global scope:
def myfunc():
global x
x = 300myfunc()print(x)
Also, use the global
keyword if you want to make a change to a global variable inside a function.
Example
To change the value of a global variable inside a function, refer to the variable by using the global
keyword:
x = 300
def myfunc():
global x
x = 200myfunc()
print(x)
Python Modules
What is a Module?
Consider a module to be the same as a code library.
A file containing a set of functions you want to include in your application.
Create a Module
To create a module just save the code you want in a file with the file extension .py
:
Example
Save this code in a file named mymodule.py
def greeting(name):
print(“Hello, “ + name)
Use a Module
Now we can use the module we just created, by using the import
statement:
Example
Import the module named mymodule, and call the greeting function:
import mymodule
mymodule.greeting(“Jonathan”)
Note: When using a function from a module, use the syntax: module_name.function_name.
Variables in Module
The module can contain functions, as already described, but also variables of all types (arrays, dictionaries, objects etc):
Example
Save this code in the file
mymodule.py
person1 = {
“name”: “John”,
“age”: 36,
“country”: “Norway”
}
Example
Import the module named mymodule, and access the person1 dictionary:
import mymodule
a = mymodule.person1[“age”]
print(a)
Naming a Module
You can name the module file whatever you like, but it must have the file extension .py
Re-naming a Module
You can create an alias when you import a module, by using the as
keyword:
Example
Create an alias for mymodule
called mx
:
import mymodule as mx
a = mx.person1[“age”]
print(a)
Built-in Modules
There are several built-in modules in Python, which you can import whenever you like.
Example
Import and use the platform
module:
import platform
x = platform.system()
print(x)
Using the dir() Function
There is a built-in function to list all the function names (or variable names) in a module. The dir()
function:
Example
List all the defined names belonging to the platform module:
import platform
x = dir(platform)
print(x)
Note: The dir() function can be used on all modules, also the ones you create yourself.
Import From Module
You can choose to import only parts from a module, by using the from
keyword.
Example
The module named mymodule
has one function and one dictionary:
def greeting(name):
print(“Hello, “ + name)person1 = {
“name”: “John”,
“age”: 36,
“country”: “Norway”
}
Example
Import only the person1 dictionary from the module:
from mymodule import person1
print (person1[“age”])
Note: When importing using the from
keyword, do not use the module name when referring to elements in the module. Example: person1["age"]
, not mymodule.person1["age"]
Python Datetime
Python Dates
A date in Python is not a data type of its own, but we can import a module named datetime
to work with dates as date objects.
Example
Import the datetime module and display the current date:
import datetime
x = datetime.datetime.now()
print(x)
Date Output
When we execute the code from the example above the result will be:
2023-03-29 18:32:44.550555
The date contains year, month, day, hour, minute, second, and microsecond.
The datetime
module has many methods to return information about the date object.
Here are a few examples, you will learn more about them later in this chapter:
Example
Return the year and name of weekday:
import datetime
x = datetime.datetime.now()
print(x.year)
print(x.strftime(“%A”))
Creating Date Objects
To create a date, we can use the datetime()
class (constructor) of the datetime
module.
The datetime()
class requires three parameters to create a date: year, month, day.
Example
Create a date object:
import datetime
x = datetime.datetime(2020, 5, 17)
print(x)
The datetime()
class also takes parameters for time and timezone (hour, minute, second, microsecond, tzone), but they are optional, and has a default value of 0
, (None
for timezone).
The strftime() Method
The datetime
object has a method for formatting date objects into readable strings.
The method is called strftime()
, and takes one parameter, format
, to specify the format of the returned string:
Example
Display the name of the month:
import datetime
x = datetime.datetime(2018, 6, 1)
print(x.strftime(“%B”))
A reference of all the legal format codes:
Directive | Description | Example | Num |
---|---|---|---|
%a | Weekday, short version | Wed | 1 |
%A | Weekday, full version | Wednesday | 2 |
%w | Weekday as a number 0-6, 0 is Sunday | 3 | 3 |
%d | Day of month 01-31 | 31 | 4 |
%b | Month name, short version | Dec | 5 |
%B | Month name, full version | December | 6 |
%m | Month as a number 01-12 | 12 | 7 |
%y | Year, short version, without century | 18 | 8 |
%Y | Year, full version | 2018 | 9 |
%H | Hour 00-23 | 17 | 10 |
%I | Hour 00-12 | 05 | 11 |
%p | AM/PM | PM | 12 |
%M | Minute 00-59 | 41 | 13 |
%S | Second 00-59 | 08 | 14 |
%f | Microsecond 000000-999999 | 548513 | 15 |
%z | UTC offset | +0100 | 16 |
%Z | Timezone | CST | 17 |
%j | Day number of year 001-366 | 365 | 18 |
%U | Week number of year, Sunday as the first day of week, 00-53 | 52 | 19 |
%W | Week number of year, Monday as the first day of week, 00-53 | 52 | 20 |
%c | Local version of date and time | Mon Dec 31 17:41:00 2018 | 21 |
%C | Century | 20 | 22 |
%x | Local version of date | 12/31/18 | 23 |
%X | Local version of time | 17:41:00 | 24 |
%% | A % character | % | 25 |
%G | ISO 8601 year | 2018 | 26 |
%u | ISO 8601 weekday (1-7) | 1 | 27 |
%V | ISO 8601 weeknumber (01-53) | 01 |
Python Math
Python has a set of built-in math functions, including an extensive math module, that allows you to perform mathematical tasks on numbers.
Built-in Math Functions
The min()
and max()
functions can be used to find the lowest or highest value in an iterable:
Example
x = min(5, 10, 25)
y = max(5, 10, 25)print(x)
print(y)
The abs()
function returns the absolute (positive) value of the specified number:
Example
x = abs(-7.25)
print(x)
The pow(x, y)
function returns the value of x to the power of y (xy).
Example
Return the value of 4 to the power of 3 (same as 4 * 4 * 4):
x = pow(4, 3)
print(x)
The Math Module
Python has also a built-in module called math
, which extends the list of mathematical functions.
To use it, you must import the math
module:
import math
When you have imported the math
module, you can start using methods and constants of the module.
The math.sqrt()
method for example, returns the square root of a number:
Example
import math
x = math.sqrt(64)
print(x)
The math.ceil()
method rounds a number upwards to its nearest integer, and the math.floor()
method rounds a number downwards to its nearest integer, and returns the result:
Example
import math
x = math.ceil(1.4)
y = math.floor(1.4)print(x) # returns 2
print(y) # returns 1
The math.pi
constant, returns the value of PI (3.14…):
Example
import math
x = math.pi
print(x)
Python JSON
JSON is a syntax for storing and exchanging data.
JSON is text, written with JavaScript object notation.
JSON in Python
Python has a built-in package called json
, which can be used to work with JSON data.
Example
Import the json module:
import json
Parse JSON – Convert from JSON to Python
If you have a JSON string, you can parse it by using the json.loads()
method.
Example
Convert from JSON to Python:
import json
# some JSON:
x = ‘{ “name”:”John”, “age”:30, “city”:”New York”}’# parse x:
y = json.loads(x)# the result is a Python dictionary:
print(y[“age”])
Convert from Python to JSON
If you have a Python object, you can convert it into a JSON string by using the json.dumps()
method.
Example
Convert from Python to JSON:
import json
# a Python object (dict):
x = {
“name”: “John”,
“age”: 30,
“city”: “New York”
}# convert into JSON:
y = json.dumps(x)# the result is a JSON string:
print(y)
You can convert Python objects of the following types, into JSON strings:
- dict
- list
- tuple
- string
- int
- float
- True
- False
- None
Example
Convert Python objects into JSON strings, and print the values:
import json
print(json.dumps({“name”: “John”, “age”: 30}))
print(json.dumps([“apple”, “bananas”]))
print(json.dumps((“apple”, “bananas”)))
print(json.dumps(“hello”))
print(json.dumps(42))
print(json.dumps(31.76))
print(json.dumps(True))
print(json.dumps(False))
print(json.dumps(None))
When you convert from Python to JSON, Python objects are converted into the JSON (JavaScript) equivalent:
Python | JSON |
---|---|
dict | Object |
list | Array |
tuple | Array |
str | String |
int | Number |
float | Number |
True | true |
False | false |
None | null |
Example
Convert a Python object containing all the legal data types:
import json
x = {
“name”: “John”,
“age”: 30,
“married”: True,
“divorced”: False,
“children”: (“Ann”,“Billy”),
“pets”: None,
“cars”: [
{“model”: “BMW 230”, “mpg”: 27.5},
{“model”: “Ford Edge”, “mpg”: 24.1}
]
}print(json.dumps(x))
Format the Result
The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.
The json.dumps()
method has parameters to make it easier to read the result:
Example
Use the indent
parameter to define the numbers of indents:
json.dumps(x, indent=4)
You can also define the separators, default value is (“, “, “: “), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:
Example
Use the separators
parameter to change the default separator:
json.dumps(x, indent=4, separators=(“. “, ” = “))
Order the Result
The json.dumps()
method has parameters to order the keys in the result:
Example
Use the sort_keys
parameter to specify if the result should be sorted or not:
json.dumps(x, indent=4, sort_keys=True)
Python RegEx
A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.
RegEx can be used to check if a string contains the specified search pattern.
RegEx Module
Python has a built-in package called re
, which can be used to work with Regular Expressions.
Import the re
module:
import re
RegEx in Python
When you have imported the re
module, you can start using regular expressions:
Example
Search the string to see if it starts with “The” and ends with “Spain”:
import re
txt = “The rain in Spain”
x = re.search(“^The.*Spain$”, txt)
RegEx Functions
The re
module offers a set of functions that allows us to search a string for a match:
Function | Description |
---|---|
findall | Returns a list containing all matches |
search | Returns a Match object if there is a match anywhere in the string |
split | Returns a list where the string has been split at each match |
sub | Replaces one or many matches with a string |
Metacharacters
Metacharacters are characters with a special meaning:
Character | Description | Example | Num |
---|---|---|---|
[] | A set of characters | “[a-m]” | 1 |
\ | Signals a special sequence (can also be used to escape special characters) | “\d” | 2 |
. | Any character (except newline character) | “he..o” | 3 |
^ | Starts with | “^hello” | 4 |
$ | Ends with | “planet$” | 5 |
* | Zero or more occurrences | “he.*o” | 6 |
+ | One or more occurrences | “he.+o” | 7 |
? | Zero or one occurrences | “he.?o” | 8 |
{} | Exactly the specified number of occurrences | “he.{2}o” | 9 |
| | Either or | “falls|stays” | 10 |
() | Capture and group | 11 |
Special Sequences
A special sequence is a \
followed by one of the characters in the list below, and has a special meaning:
Character | Description | Example | Num |
---|---|---|---|
\A | Returns a match if the specified characters are at the beginning of the string | “\AThe” | 1 |
\b | Returns a match where the specified characters are at the beginning or at the end of a word (the “r” in the beginning is making sure that the string is being treated as a “raw string”) |
r”\bain” r”ain\b” |
2 |
\B | Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the “r” in the beginning is making sure that the string is being treated as a “raw string”) |
r”\Bain” r”ain\B” |
3 |
\d | Returns a match where the string contains digits (numbers from 0-9) | “\d” | 4 |
\D | Returns a match where the string DOES NOT contain digits | “\D” | 5 |
\s | Returns a match where the string contains a white space character | “\s” | 6 |
\S | Returns a match where the string DOES NOT contain a white space character | “\S” | 7 |
\w | Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character) | “\w” | 8 |
\W | Returns a match where the string DOES NOT contain any word characters | “\W” | 9 |
\Z | Returns a match if the specified characters are at the end of the string | “Spain\Z” | 10 |
Sets
A set is a set of characters inside a pair of square brackets []
with a special meaning:
Set | Description | Num |
---|---|---|
[arn] | Returns a match where one of the specified characters (a , r , or n ) is present |
01 |
[a-n] | 02 | |
[^arn] | Returns a match for any character EXCEPT a , r , and n |
03 |
[0123] | Returns a match where any of the specified digits (0 , 1 , 2 , or 3 ) are present |
04 |
[0-9] | 05 | |
[0-5][0-9] | Returns a match for any two-digit numbers from 00 and 59 |
06 |
[a-zA-Z] | Returns a match for any character alphabetically between a and z , lower case OR upper case |
07 |
[+] | In sets, + , * , . , | , () , $ ,{} has no special meaning, so [+] means: return a match for any + character in the string |
08 |
The findall() Function
The findall()
function returns a list containing all matches.
Example
Print a list of all matches:
import re
txt = “The rain in Spain”
x = re.findall(“ai”, txt)
print(x)
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned:
Example
Return an empty list if no match was found:
import re
txt = “The rain in Spain”
x = re.findall(“Portugal”, txt)
print(x)
The search() Function
The search()
function searches the string for a match, and returns a Match object if there is a match.
If there is more than one match, only the first occurrence of the match will be returned:
Example
Search for the first white-space character in the string:
import re
txt = “The rain in Spain”
x = re.search(“\s”, txt)print(“The first white-space character is located in position:”, x.start())
If no matches are found, the value
None
is returned:
Example
Make a search that returns no match:
import re
txt = “The rain in Spain”
x = re.search(“Portugal”, txt)
print(x)
The split() Function
The split()
function returns a list where the string has been split at each match:
Example
Split at each white-space character:
import re
txt = “The rain in Spain”
x = re.split(“\s”, txt)
print(x)
You can control the number of occurrences by specifying the maxsplit
parameter:
Example
Split the string only at the first occurrence:
import re
txt = “The rain in Spain”
x = re.split(“\s”, txt, 1)
print(x)
The sub() Function
The sub()
function replaces the matches with the text of your choice:
Example
Replace every white-space character with the number 9:
import re
txt = “The rain in Spain”
x = re.sub(“\s”, “9”, txt)
print(x)
You can control the number of replacements by specifying the count
parameter:
Example
Replace the first 2 occurrences:
import re
txt = “The rain in Spain”
x = re.sub(“\s”, “9”, txt, 2)
print(x)
Match Object
A Match Object is an object containing information about the search and the result.
Note: If there is no match, the value
None
will be returned, instead of the Match Object.
Example
Do a search that will return a Match Object:
import re
txt = “The rain in Spain”
x = re.search(“ai”, txt)
print(x) #this will print an object
The Match object has properties and methods used to retrieve information about the search, and the result:
.span()
returns a tuple containing the start-, and end positions of the match.
.string
returns the string passed into the function
.group()
returns the part of the string where there was a match
Example
Print the position (start- and end-position) of the first match occurrence.
The regular expression looks for any words that starts with an upper case “S”:
import re
txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.span())
Example
Print the string passed into the function:
import re
txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.string)
Example
Print the part of the string where there was a match.
The regular expression looks for any words that starts with an upper case “S”:
import re
txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.group())
Note: If there is no match, the value
None
will be returned, instead of the Match Object.
Python PIP
What is PIP?
PIP is a package manager for Python packages, or modules if you like.
Note: If you have Python version 3.4 or later, PIP is included by default.
What is a Package?
A package contains all the files you need for a module.
Modules are Python code libraries you can include in your project.
Check if PIP is Installed
Navigate your command line to the location of Python’s script directory, and type the following:
Example
Check PIP version:
C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip –version
Install PIP
If you do not have PIP installed, you can download and install it from this page: https://pypi.org/project/pip/
Download a Package
Downloading a package is very easy.
Open the command line interface and tell PIP to download the package you want.
Navigate your command line to the location of Python’s script directory, and type the following:
Example
Download a package named “camelcase”:
C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip install camelcase
Now you have downloaded and installed your first package!
Using a Package
Once the package is installed, it is ready to use.
Import the “camelcase” package into your project.
Example
Import and use “camelcase”:
import camelcase
c = camelcase.CamelCase()
txt = “hello world”
print(c.hump(txt))
Find Packages
Find more packages at https://pypi.org/.
Remove a Package
Use the uninstall
command to remove a package:
Example
Uninstall the package named “camelcase”:
C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip uninstall camelcase
The PIP Package Manager will ask you to confirm that you want to remove the camelcase package:
Uninstalling camelcase-02.1:
Would remove:
c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camelcase-0.2-py3.6.egg-info
c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camelcase\*
Proceed (y/n)?
Press y
and the package will be removed.
List Packages
Use the list
command to list all the packages installed on your system:
Example
List installed packages:
C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip list
Result:
Package Version
———————–
camelcase 0.2
mysql-connector 2.1.6
pip 18.1
pymongo 3.6.1
setuptools 39.0.1
Python Try Except
The try
block lets you test a block of code for errors.
The except
block lets you handle the error.
The else
block lets you execute code when there is no error.
The finally
block lets you execute code, regardless of the result of the try- and except blocks.
Exception Handling
When an error occurs, or exception as we call it, Python will normally stop and generate an error message.
These exceptions can be handled using the try
statement:
Example
The try
block will generate an exception, because x
is not defined:
try:
print(x)
except:
print(“An exception occurred”)
Since the try block raises an error, the except block will be executed.
Without the try block, the program will crash and raise an error:
Example
This statement will raise an error, because x
is not defined:
print(x)
Many Exceptions
You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:
Example
Print one message if the try block raises a NameError
and another for other errors:
try:
print(x)
except NameError:
print(“Variable x is not defined”)
except:
print(“Something else went wrong”)
Else
You can use the else
keyword to define a block of code to be executed if no errors were raised:
Example
In this example, the try
block does not generate any error:
try:
print(“Hello”)
except:
print(“Something went wrong”)
else:
print(“Nothing went wrong”)
Finally
The finally
block, if specified, will be executed regardless if the try block raises an error or not.
Example
try:
print(x)
except:
print(“Something went wrong”)
finally:
print(“The ‘try except’ is finished”)
This can be useful to close objects and clean up resources:
Example
Try to open and write to a file that is not writable:
try:
f = open(“demofile.txt”)
try:
f.write(“Lorum Ipsum”)
except:
print(“Something went wrong when writing to the file”)
finally:
f.close()
except:
print(“Something went wrong when opening the file”)
The program can continue, without leaving the file object open.
Raise an exception
As a Python developer you can choose to throw an exception if a condition occurs.
To throw (or raise) an exception, use the raise
keyword.
Example
Raise an error and stop the program if x is lower than 0:
x = –1
if x < 0:
raise Exception(“Sorry, no numbers below zero”)
The raise
keyword is used to raise an exception.
You can define what kind of error to raise, and the text to print to the user.
Example
Raise a TypeError if x is not an integer:
x = “hello”
if not type(x) is int:
raise TypeError(“Only integers are allowed”)
Python User Input
User Input
Python allows for user input.
That means we are able to ask the user for input.
The method is a bit different in Python 3.6 than Python 2.7.
Python 3.6 uses the input()
method.
Python 2.7 uses the raw_input()
method.
The following example asks for the username, and when you entered the username, it gets printed on the screen:
Python 3.6
username = input(“Enter username:”)
print(“Username is: “ + username)
Python 2.7
username = raw_input(“Enter username:”)
print(“Username is: “ + username)
Python stops executing when it comes to the input()
function, and continues when the user has given some input.
Python String Formatting
To make sure a string will display as expected, we can format the result with the format()
method.
String format()
The format()
method allows you to format selected parts of a string.
Sometimes there are parts of a text that you do not control, maybe they come from a database, or user input?
To control such values, add placeholders (curly brackets {}
) in the text, and run the values through the format()
method:
Example
Add a placeholder where you want to display the price:
price = 49
txt = “The price is {} dollars”
print(txt.format(price))
You can add parameters inside the curly brackets to specify how to convert the value:
Example
Format the price to be displayed as a number with two decimals:
txt = “The price is {:.2f} dollars”
Multiple Values
If you want to use more values, just add more values to the format() method:
print(txt.format(price, itemno, count))
And add more placeholders:
Example
quantity = 3
itemno = 567
price = 49
myorder = “I want {} pieces of item number {} for {:.2f} dollars.”
print(myorder.format(quantity, itemno, price))
Index Numbers
You can use index numbers (a number inside the curly brackets {0}
) to be sure the values are placed in the correct placeholders:
Example
quantity = 3
itemno = 567
price = 49
myorder = “I want {0} pieces of item number {1} for {2:.2f} dollars.”
print(myorder.format(quantity, itemno, price))
Also, if you want to refer to the same value more than once, use the index number:
Example
age = 36
name = “John”
txt = “His name is {1}. {1} is {0} years old.”
print(txt.format(age, name))
Named Indexes
You can also use named indexes by entering a name inside the curly brackets {carname}
, but then you must use names when you pass the parameter values txt.format(carname = "Ford")
:
Example
myorder = “I have a {carname}, it is a {model}.”
print(myorder.format(carname = “Ford”, model = “Mustang”))