How Can I Modify Non-Global Variables in Outer Scopes in Python?

Modifying Non-Global Variables in Outer Scope in Python

In Python, variables within a function typically belong to the local scope, unless explicitly declared as global. However, it is sometimes necessary to modify variables defined in an outer (enclosing) but non-global scope. This question explores how to achieve this.

Given the example code:

def A():
    b = 1
    def B():
        # Access to 'b' is possible here.
        print(b)
        # Direct modification of 'b' fails.
    B()
A()

The variable b in function B resides in a non-global, enclosing scope. Attempts to modify b directly result in an UnboundLocalError. The global keyword can't be used since b is not declared at the global level.

Python 3 Solution:

Nonlocal Scope (Python 3.x) can be used to solve this issue:

def A():
    b = 1
    def B():
        nonlocal b  # Nonlocal keyword
        b = 2
    B()
    print(b)  # Output: 2
A()

Python 2 Solution:

Mutable Objects (Python 2.x):

Instead of reassigning variables directly, use mutable objects (e.g., lists, dicts) and mutate their values:

def A():
    b = []
    def B():
        b.append(1)  # Mutation of 'b'
    B()
    B()
    print(b)  # Output: [1, 1]
A()

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