Python Variables

last modified February 26, 2025

Variables in Python are used to store data that can be referenced and manipulated throughout a program. Python supports various types of variables, including bound/unbound, static, class, and instance variables. This tutorial covers the different contexts in which variables are used, along with practical examples.

Variables are names that refer to values stored in memory. They allow us to store and manipulate data in our program. Python variables do not require explicit declaration and can change types dynamically. Variables can be categorized based on their scope and context, such as local, global, class, and instance variables.

Module variables

In Python, module variables are variables that are defined at the top level of a module (a .py file). These variables can be accessed from any part of the module and can also be imported into other modules.

Key Characteristics:

Global within the Module: Module variables are global within the module in which they are defined. They can be accessed by any function or class within that module.
Module Scope: They have a module scope, meaning they are not confined to any specific function or class but are accessible throughout the entire module.
Importable: Module variables can be imported into other modules using the import statement.

Let's consider a module named example_module.py:

example_module.py

# Module variables 
PI = 3.14159 
E = 2.71828

# Function that uses module variables 
def calculate_circle_area(radius): 
    return PI * radius * radius

# Class that uses module variables 
class MathConstants: 

    def get_constants(self):
        return PI, E

In this example PI and E are module variables. The function calculate_circle_area and the method get_constants use these module variables.

We can import these module variables into another module or script:

main.py

from example_module import PI, E

print(f'The value of PI is: {PI}') 
print(f'The value of E is:{E}')

In this case, PI and E are imported from example_module and can be used in another_module.

Bound and Unbound Variables

This example demonstrates bound and unbound variables in Python.

bound_unbound.py

x = 10  # Bound variable (assigned a value)

def my_function():
    print(y)  # Unbound variable (not assigned a value)

# my_function()  # Uncommenting this line will raise a NameError

A bound variable is one that has been assigned a value, like x. An unbound variable is one that has not been assigned a value, like y in my_function. Accessing an unbound variable raises a NameError.

Function-scoped Variables

This example demonstrates Function-scoped variables in Python.

static_variable.py

def my_function():
    if not hasattr(my_function, "counter"):
        my_function.counter = 0  # Static variable
    my_function.counter += 1
    return my_function.counter

print(my_function())  # Output: 1
print(my_function())  # Output: 2
print(my_function())  # Output: 3

A function-scoped variable retains its state across function calls.

Class Variables

This example demonstrates class variables in Python.

class_variable.py

class MyClass:
    class_var = 10  # Class variable

print(MyClass.class_var)  # Output: 10

obj1 = MyClass()
obj2 = MyClass()

print(obj1.class_var)  # Output: 10
print(obj2.class_var)  # Output: 10

MyClass.class_var = 20  # Modify class variable
print(obj1.class_var)  # Output: 20
print(obj2.class_var)  # Output: 20

A class variable is a variable that is shared by all instances of a class. It is defined within the class but outside any method. Class variables are accessed using the class name or an instance of the class.

Instance Variables

This example demonstrates instance variables in Python.

instance_variable.py

class MyClass:
    def __init__(self, value):
        self.instance_var = value  # Instance variable

obj1 = MyClass(10)
obj2 = MyClass(20)

print(obj1.instance_var)  # Output: 10
print(obj2.instance_var)  # Output: 20

obj1.instance_var = 30
print(obj1.instance_var)  # Output: 30
print(obj2.instance_var)  # Output: 20

An instance variable is a variable that is unique to each instance of a class. It is defined within the __init__ method or other instance methods. Instance variables are accessed using the instance name.

Global Variables

This example demonstrates global variables in Python.

global_variable.py

x = 10  # Global variable

def my_function():
    global x  # Declare x as global
    x = 20
    print(f"Inside function: {x}")

my_function()
print(f"Outside function: {x}")

# Output:
# Inside function: 20
# Outside function: 20

A global variable is a variable that is defined outside any function or class and can be accessed throughout the program. To modify a global variable inside a function, use the global keyword.

Local Variables

This example demonstrates local variables in Python.

local_variable.py

def my_function():
    x = 10  # Local variable
    print(f"Inside function: {x}")

my_function()
# print(x)  # Uncommenting this line will raise a NameError

# Output:
# Inside function: 10

A local variable is a variable that is defined within a function or block and is only accessible within that scope. Attempting to access a local variable outside its scope raises a NameError.

Best Practices for Using Variables

Use Descriptive Names: Choose meaningful variable names to improve code readability.
Limit Scope: Declare variables in the smallest scope possible to avoid unintended side effects.
Avoid Global Variables: Minimize the use of global variables to reduce complexity and potential bugs.
Use Constants for Fixed Values: Use uppercase names for constants to indicate that their values should not change.

Source

Python Scopes and Namespaces Documentation

In this article, we have explored Python variables and demonstrated their usage in different contexts through practical examples.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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