How Can I Modify Variables in Enclosing Scopes in Python?

Modifying Variables in Enclosing Scope: A Pythonesque Conundrum

In Python, the concept of scope governs the visibility and accessibility of variables. When dealing with nested and enclosing scopes, it's not always straightforward to modify variables in an outer scope.

The Problem: UnboundLocalError

Consider the following code snippet:

def A():
    b = 1
    def B():
        # Access 'b', no problem
        print(b)
        # Attempt to modify 'b'
        b = 2  # UnboundLocalError
    B()
A()

Here, the variable b is defined in the enclosing scope of B, but it's not global. Trying to modify b directly within B results in an UnboundLocalError because b is not declared as a local variable in B.

The Solution: Embrace the Non-Global Scope

In Python 3, the nonlocal keyword comes to the rescue. It allows you to modify variables in an enclosing, non-global scope.

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

Python 2's Alternative: Mutability to the Rescue

While Python 3 has the convenient nonlocal keyword, Python 2 doesn't provide a direct solution. A workaround is to use mutable objects (e.g., lists or dictionaries) and mutate their values instead of reassigning variables.

def foo():
    a = []
    def bar():
        a.append(1)
    bar()
    bar()
    print(a)  # Output: [1, 1]

By manipulating the list a, which is an object, you can essentially modify its content while adhering to Python 2's scope rules.

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