Why Do I Get NameError or UnboundLocalError When Handling Exceptions in Python 3.x?

Understanding the NameError and UnboundLocalError Issues with Named Exceptions

In Python 2.x, the code snippet below appropriately printed the value of the named exception, exc, outside the except block:

<code class="python">exc = None
try:
    raise Exception
except Exception as exc:
    pass
print(exc)</code>

However, in Python 3.x, attempting the same code yields a NameError (or UnboundLocalError in a function context). To rectify this issue, it's essential to grasp the rationale behind Python's updated behavior.

Python 3.x Exception Scope and Cleanup

The except statement in Python 3.x explicitly restricts the scope of the bound exception to prevent circular references and premature garbage collection. When an exception is assigned using the as syntax, it gets cleared at the end of the except clause. This necessitates assigning the exception to a different name outside the except block for further access.

Simply re-assigning exc = exc will not suffice because the except clause does not create a new scope. Instead, it removes the name specified in the exception assignment from the current one.

Historical Context and Python 2.x Behavior

In Python 2.x, exceptions did not possess a reference to the traceback, eliminating the need for such stringent cleanup as in Python 3.x. The absence of circular references allowed exceptions to be bound to named variables outside the except block without causing issues.

Recommendations for Handling Exceptions

To effectively handle exceptions in Python 3.x, the following guidelines are recommended:

1. Bind the exception to a new variable outside the except clause.
2. If necessary, explicitly clear the traceback reference to prevent circular references and facilitate garbage collection.

The following code exemplifies these recommendations:

<code class="python">try:
    raise Exception("foo")
except Exception as e:
    exc = e  # Bind to a new variable
    exc.__traceback__ = None  # Explicitly clear traceback</code>

By adhering to these practices, you can confidently handle and access exceptions in Python 3.x, avoiding the pitfalls posed by NameError and UnboundLocalError.

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