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
- data analytics and data science
- database administration
Introduction
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
Let's get started!
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
Python will ignore comments starting with a #
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(I love you Mary!)
Multi-line comments
Python does not really have a syntax for multi line comments.
To add a multiline comment you could insert a # for each line.
Example:
#This is a comment
#written
#in more than one line
print("Hello World!")
You can also use string literals.
Python will ignore any code written in between them
Example:
"""
This is a comment
written
in more than one line
""""
Python Variables
Variables
Variables are used to store 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)
5
John
Variables do not need to be declared with any particular type, and can even change type after they have been set.
Example:
x = 4 #This is a of type int
x = "Sally" # This is of type str
print(x)
Sally
Casting
If you want to specify the data type of a variable, this can be done with casting.
Example:
x = str(2) # x will be '2'
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))
type int
type str
Case-sensitive
Variable names are case-sensitive
Example:
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 variable names:
- 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)
Example:
Legal variable names:
myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"
Camel Case
Camel case is the most popular form of declaring multi word variable names. These techniques are use because multi word variable names can be difficult to read
Each word except the first starts with a capital letter
Example:
myVeryFirstVariableName = "John"
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", "Apple"
print(x)
print(y)
print(z)
Orange
Banana
Apple
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)
Orange
Orange
Orange
Unpack a Collection
If you have a collection of values in a list, tuple etc. Python allows you extract the values into variables. This is called unpacking.
Example:
fruits = ["orange", "banana", "cherry"]
x, y, z = fruits
print(x)
print(y)
print(z)
orange
banana
cherry
Python Output Variables
Output Variables
The Python print statement is often used to output variables.
To combine both text and a variable, Python uses the + character:
Example:
x = "awesome"
print("Python is + x")
Python is awesome
You can also use the + character to add a variable to another variable:
Example:
x = "Python "
y = "is awesome"
z = x + y
print(z)
Python is awesome
For numbers, the + character works as a mathematical operator
Example
x = 5
y = 10
print(x + y)
15
Please Note.If you try to combine a string and a number, Python will give you an error:
Example:
x = 5
y = "John"
print(x + y)
TypeError: unsupported operand type(s) for +: 'int' and 'str'
Let's turn it up a notch
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()
Python is awesome
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)
Python is fantastic
Python is awesome
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)
Python is awesome
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)
That's all for today. Next time we shall continue with Data types and other material as we continue to dive deeper into one of the most popular programming languages
Reference
Most of the the information on this project page has been taken from the W3 Schools page on Python Variables.
Credits
Built by Richard Orido for the freeCodeCamp Technical Documentation Page project.