12 October 2020

Python Variables - How to Declare and Use Variables in Python

development

A guide on how to create and use variables in Python.

Python Variables

Variables are named locations that are used to store references to the object stored in memory.

When we create variables in Python, we must consider the following rules:

A variable name must start with a letter or underscore
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 (date, Date and DATE are three different variables)
Variables can be of any length
Variable names cannot be Python keywords

Python Keywords

False      class      finally    is         return
None       continue   for        lambda     try
True       def        from       nonlocal   while
and        del        global     not        with
as         elif       if         or         yield
pass       else       import     assert
break      except     in         raise

Assigning Values to Variables

We use the assignment operator = to assign value to a variable.

Example valid and invalid variable names and assignments:

#Legal variable names:
name = "John"
error_404 = 404
_status_200 = "OK"
mySurname = "Doe"
SURNAME = "Doe"
surname2 = "Doe"

#Illegal variable names:
200_OK = 200
error-500 = "Server Error"
my var = "John"
$myname = "John"

Note: in Python, you don't need to declare types of variables ahead of time. The interpreter automatically detects the type of the variable by the data it contains.

Multiple Assignments

In Python, we can assign values to multiple variables in one line:

Example:

ok, redirect, server_error = 200, 300, 500
print(ok)
print(redirect)
print(server_error)

Output:

200
300
500

We can also assign the same value to multiple variables:

err_500 = err_501 = err_502 = "server_error"
print(err_500)
print(err_501)
print(err_502)

Global Variables

Variables that are defined outside of a function are known as global variables.

Global variables can be used both inside and outside of functions.

status_ok = 200

def status_code():
    print("Status code is ", status_ok)

status_code()

If you create a variable with the same name inside a function, then the variable will be local to the function. The global variable will keep its value as when it was declared.

Example:

status = 200

def status_code():
    status = 401
    print("Status code is ", status)

status_code()

print("Status code is ", status)

Output:

Status code is  401 // first print statement
Status code is  200 // second print statement

If you require to change the global variable’s value inside of a function, you have to use the global keyword.

For example:

status = 200

def status_code():
    global status
    status = 401
    print("Status code is ", status)

status_code()

print("Status code is ", status)

Output

Status code is  401 // first print statement
Status code is  401 // second print statement

12 October 2020