How to Resolve UnboundLocalError in Nested Function Scopes in Python?

UnboundLocalError in Nested Function Scopes

In Python, accessing a variable defined in an outer function from a nested function can sometimes result in an UnboundLocalError. Consider the following example:

<code class="python">def outer():
    ctr = 0

    def inner():
        ctr += 1

    inner()</code>

Running this code will raise an UnboundLocalError for the variable ctr in the inner function. This error occurs because Python treats ctr as a local variable within the inner function, even though it's defined in the outer function. To resolve this issue, we need to use a mechanism that allows the inner function to access the outer function's scope.

Solution:

Python 3 introduced the nonlocal statement, which permits nonlocal variable modification. By adding nonlocal to the inner function, we explicitly declare ctr as a nonlocal variable, allowing its rebinding within the inner function.

<code class="python">def outer():
    ctr = 0

    def inner():
        nonlocal ctr
        ctr += 1

    inner()</code>

Alternatively, in Python 2, which lacks the nonlocal statement, we can work around this issue by enclosing the counter variable within a list or other data structure to avoid barename rebinding:

<code class="python">ctr = [0]

def inner():
    ctr[0] += 1</code>

This approach maintains the value of ctr within the list ctr, preventing the UnboundLocalError from occurring.

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