How does the scope of named exceptions differ between Python 2.x and 3.x?

Exception Scope and Assigning Bound Exceptions in Python

In Python, it's essential to understand the scope of named exceptions after the except block concludes. This becomes apparent when comparing Python 2.x and 3.x behavior.

Python 3.x Behavior: NameError and UnboundLocalError

In Python 3.x, attempting to access an exception named in an except block outside that block results in either a NameError (if accessed outside a function) or UnboundLocalError (if accessed inside a function).

Reason for the Change

This change is due to Python 3.x's decision to limit the scope of named exceptions explicitly within the except clause. This is to prevent circular references and avoid the need to clean up tracebacks explicitly.

Workaround

To access an exception outside the except block, it's necessary to rebind it to a new name. For example:

<code class="python">exc = None
try:
    raise Exception
except Exception as exc:
    exc = exc # Rebinding to a new name</code>

Explicit Traceback Clearing

If desired, it's possible to clear the traceback explicitly after rebinding the exception:

<code class="python">exc = None
try:
    raise Exception("foo")
except Exception as e:
    exc = e
    exc.__traceback__ = None</code>

Contrast with Python 2.x

In Python 2.x, exceptions did not contain references to their traceback, so this scope limitation was not necessary. As a result, assigning the exception to a variable after the except block had no negative consequences.

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