Variable Scoping

Status

This note is complete, reviewed, and considered stable.

Variable scoping in Python determines the accessibility of variables within different parts of the code. It defines where a variable can be read or modified. Python follows specific scoping rules, which are influenced by the LEGB rule (Local, Enclosing, Global, Built-in). Understanding scoping is crucial to avoid conflicts and unexpected behavior in your programs.

Scope of Variables

1. Local Scope

   • Variables defined inside a function are said to have a local scope.
   • These variables are accessible only within the function where they are declared.

   Example:

   def my_function():
       x = 10  # Local variable
       print(x)
   
   my_function()
   # print(x)  # Error: x is not accessible outside the function

2. Enclosing Scope

   • This refers to variables in the scope of enclosing (outer) functions in a nested function structure.
   • These variables are neither local nor global to the inner function but are in its enclosing scope.

   Example:

   def outer_function():
       x = "Outer variable"
   
       def inner_function():
           print(x)  # Accessing enclosing variable
   
       inner_function()
   
   outer_function()

3. Global Scope

   • Variables defined at the top level of a script or module have a global scope.
   • They can be accessed and modified throughout the module unless shadowed by a local variable.

   Example:

   x = "Global variable"
   
   def my_function():
       print(x)  # Accessing global variable
   
   my_function()

4. Built-in Scope

   • This refers to Python’s built-in functions and names (e.g., print, len, type).
   • These names are always accessible unless overridden.

   Example:

   print(len([1, 2, 3]))  # Built-in function

The LEGB Rule

Python resolves variable names using the LEGB rule:

1. Local: Checks the innermost scope (inside the current function).
2. Enclosing: Checks the scope of any enclosing functions.
3. Global: Checks the global (module-level) scope.
4. Built-in: Checks Python’s built-in namespace.

Example:

x = "Global"

def outer_function():
    x = "Enclosing"

    def inner_function():
        x = "Local"
        print(x)  # Prints "Local"

    inner_function()

outer_function()

Modifying Variables in Different Scopes

1. Using global Keyword

• If we want to modify a global variable inside a function, use the global keyword.

Example:

x = 10

def modify_global():
    global x
    x = 20  # Modifies the global variable

modify_global()
print(x)  # Output: 20

2. Using nonlocal Keyword

• To modify a variable in an enclosing scope, use the nonlocal keyword.

Example:

def outer_function():
    x = "Enclosing"

    def inner_function():
        nonlocal x
        x = "Modified Enclosing"

    inner_function()
    print(x)  # Output: Modified Enclosing

outer_function()

Shadowing of Variables

• A variable in a local scope can shadow (override) variables in an outer or global scope with the same name.

Example:

x = "Global"

def my_function():
    x = "Local"  # Shadows the global variable
    print(x)

my_function()  # Output: Local
print(x)       # Output: Global

Dynamic Nature of Python Scoping

1. Python allows the declaration of variables dynamically at runtime.
   Example:

   def dynamic_example():
       if True:
           x = 100  # x exists only within the function
       print(x)  # Output: 100
   
   dynamic_example()

2. Variables declared in if, for, and while blocks are still accessible within the function (unlike some languages like C++ or Java).

Best Practices for Variable Scoping

1. Avoid Overusing global and nonlocal:

   • Use global and nonlocal sparingly as they make code harder to read and debug.

2. Use Descriptive Names:

   • Use meaningful variable names to reduce the risk of unintended shadowing.

3. Minimize Global Variables:

   • Restrict the use of global variables as they can lead to unexpected behavior in larger programs.

4. Encapsulate Logic in Functions:

   • Encapsulation promotes the use of local scope and reduces conflicts.