Intro to Python Variables

Learning objective: By the end of this lesson, students will be able to define and reassign Python variables.

Python variables

Variables in Python hold the data an app needs when it runs, much like they do in JavaScript.

Declaring

Here is how we declare a variable in Python:

num = 15

Notice the lack of let or const keywords. In Python, we only need to decide on a variable’s name and the value we want to assign to it. There is no way to define a constant variable in Python, but there are conventions to help us communicate that a variable should be a constant, which we’ll cover soon.

You cannot declare a variable without assigning to it:

movie
# returns: NameError: name 'movie' is not defined
# this is illegal syntax that cannot be used

This is because there is no undefined data type in Python.

Naming convention

Variables are case sensitive - num and Num would be two different variables.

When you have a variable name with multiple words, the convention is to snake_case the identifier:

# in Python, we use snake_case
my_number = 10

Reassigning variables

We can freely assign a new value to a variable after it has been declared:

my_number = 15
print(my_number)
# prints: 15
my_number = -4
print(my_number)
# prints: -4

Reassignment replaces the current value, and the previous value is lost, just like in JavaScript.

Constants

While Python doesn’t have a way to declare constants, the convention is to declare any variable you’d like to be known as a constant in SCREAMING_SNAKE_CASE.

MY_FAVORITE_NUMBER = 5

⚠️ This convention does not prevent reassignment. It only communicates that the variable is not intended to be reassigned.