How to Handle Unbound Local Variable Errors in Nested Function Scopes?

Resolving UnboundLocalError in Nested Function Scopes

The Python interpreter encounters an UnboundLocalError when accessing an unbound local variable within a nested function. This issue arises when a nested function attempts to modify a variable declared within the outer function.

Example:

Consider the following code:

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

    def inner():
        ctr += 1

    inner()</code>

Upon executing this code, the interpreter generates the following error:

Traceback (most recent call last):
  File "foo.py", line 9, in <module>
    outer()
  File "foo.py", line 7, in outer
    inner()
  File "foo.py", line 5, in inner
    ctr += 1
UnboundLocalError: local variable 'ctr' referenced before assignment

Cause:

Despite having nested scopes, the inner function cannot access the 'ctr' variable directly because it is defined in the outer function. This results in an unbound variable, triggering the UnboundLocalError.

Solution:

Python 3 offers the 'nonlocal' statement to enable variable rebinding in nested scopes. Modifying the code to include 'nonlocal' resolves the issue:

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

    def inner():
        nonlocal ctr
        ctr += 1

    inner()</code>

For Python 2 users, alternative approaches are necessary:

• Barename Removal:
  Remove the bare ctr variable and place it within a data structure or as an attribute. For instance, ctr = [0].
• Variable Reassignment:
  Reassign the ctr variable within the inner function instead of using barename rebinding: ctr = ctr 1.

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