Python Variables

Python Variables: Memory Model, Naming, & References

Variables in Python are dynamically typed reference labels bound to objects stored in heap memory. Assigning a variable does not copy data; it creates a reference to an underlying object.

Python Variable Reference Model

flowchart LR
    subgraph Stack Scope
        x["x"]
        y["y"]
    end
    subgraph Heap Memory
        obj1["int Object: 42"]
        obj2["list Object: [1, 2, 3]"]
    end
    x --> obj1
    y --> obj2

Naming Rules & Conventions (PEP 8)

• Must begin with a letter or underscore _.
• Case-sensitive (totalAmount and total_amount are distinct).
• Use snake_case for variables and function names (user_session_token).
• Use UPPER_SNAKE_CASE for module-level constants (MAX_RETRY_ATTEMPTS = 5).

Practical Code Example

from typing import List

# Constant definition
MAX_REQUEST_LIMIT: int = 100

def demonstrate_variable_references() -> None:
    # Value assignment & object identity (id())
    a: int = 500
    b: int = a  # 'b' points to the same integer object in heap
    
    print(f"a = {a}, id(a) = {id(a)}")
    print(f"b = {b}, id(b) = {id(b)}")
    print(f"a is b: {a is b}")
    
    # Reassigning 'a' creates a reference to a new integer object
    a = 600
    print(f"After reassigning a -> 600: a = {a}, b = {b}")
    print(f"a is b: {a is b}")

    # Mutable object behavior
    list_one: List[int] = [1, 2, 3]
    list_two: List[int] = list_one
    list_two.append(4)
    print(f"Modified list_two affects list_one: {list_one}")

if __name__ == "__main__":
    demonstrate_variable_references()

Best Practices & Gotchas

• is vs ==: == checks value equality (are contents equal?), whereas is checks object identity (do variables reference the exact same memory address?).
• Small Integer Caching: CPython caches integers between -5 and 256 in memory for performance optimization.
• Unpack Variables Cleanly: Use sequence unpacking x, y = 10, 20 or extended unpacking first, *rest = [1, 2, 3, 4].

Self-Check Challenge

Create two variables containing identical lists [10, 20]. Compare them using both == and is. Print and explain why == returns True while is returns False.