Global and nonlocal exception aliases are removed from scope

[noahbkim]

Bug report

Bug description:

When you alias an exception caught in except to a global or nonlocal name, that name will be removed from its original scope. For global:

error = None

def f():
    global error
    try:
        raise ValueError()
    except ValueError as error:
        pass

# Output
error before: None
Traceback (most recent call last):
  File "test.py", line 14, in <module>
    print(f"error after: {error}")
                          ^^^^^
NameError: name 'error' is not defined. Did you mean: 'OSError'?

For nonlocal:

def g():
    error = None

    def f():
        nonlocal error
        try:
            raise ValueError()
        except ValueError as error:
            pass

    print(f"error before: {error}")
    f()
    print(f"error after: {error}")

g()

# Output
error before: None
Traceback (most recent call last):
  File "test.py", line 16, in <module>
    g()
  File "test.py", line 13, in g
    print(f"error after: {error}")
                          ^^^^^
UnboundLocalError: cannot access local variable 'error' where it is not associated with a value

This is a bit counterintuitive. Is it intended behavior for any reason?

CPython versions tested on:

3.12

Operating systems tested on:

Linux

[ronaldoussoren]

The behaviour is described in the language reference. PEP 3110 explains why the feature was added, basically to avoid reference loops between the caught exception and the current frame that need the GC to clear up when using local variables.

[noahbkim]

Thanks for the quick response. While it'd be nice for explicitly global/nonlocal names to be exempt from this rule, it's such a niche case that I'm happy with "working as intended".