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Unexpected location of naked exception wrapped in except* block #128799

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jobh opened this issue Jan 13, 2025 · 6 comments · May be fixed by #128971
Open

Unexpected location of naked exception wrapped in except* block #128799

jobh opened this issue Jan 13, 2025 · 6 comments · May be fixed by #128971
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@iritkatriel
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interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@jobh
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jobh commented Jan 13, 2025 •
edited by bedevere-app bot
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Bug report

Bug description:

When an ExceptionGroup is implicitly constructed by catching a naked exception in an except* block, the traceback of the ExceptionGroup seems to point to the stackframe above where it is logically created:

def f():
    try:
        raise Exception  # Exception (inner) traceback terminates here
    except* Exception as e:
        raise

f()  # ExceptionGroup (wrapper) traceback terminates here

→

  + Exception Group Traceback (most recent call last):
  |   File "/home/jobh/src/hypothesis/test_naked_exception.py", line 7, in <module>
  |     f()  # ExceptionGroup (wrapper) traceback terminates here
  |     ^^^
  | ExceptionGroup:  (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/home/jobh/src/hypothesis/test_naked_exception.py", line 3, in f
    |     raise Exception  # Exception (inner) traceback terminates here
    |     ^^^^^^^^^^^^^^^
    | Exception
    +------------------------------------

I'm not sure if this should be called a bug, as it's not specifically documented how the wrapping ExceptionGroup traceback is constructed. But it is surprising, and did cause a bug in hypothesis where the Exception traceback is used to detect whether an exception originates in hypothesis itself or in user code 12. The issue being that for wrapped naked exceptions we have to inspect the inner exception since the outer exception appears to be raised by the calling function.

As an alternative to the current behaviour, would it be possible to create the wrapper ExceptionGroup with a traceback pointing to where it is logically triggered - i.e., the except* block - rather than initially empty? IMO, that would be less suprising and would allow us to use the traceback without special-casing ExceptionGroup.

([edit] Or, if the desire is to avoid branching the traceback between the wrapper and the inner exception, use the outermost frame of the naked exception as the initial ExceptionGroup traceback. This would still avoid the appearance of happening in the caller.)

CPython versions tested on:

3.11, 3.12, 3.13

Operating systems tested on:

Linux

Linked PRs

Footnotes

  1. https://github.com/HypothesisWorks/hypothesis/issues/4183 ↩

  2. https://github.com/HypothesisWorks/hypothesis/pull/4239 ↩
@jobh jobh added the type-bug An unexpected behavior, bug, or error label Jan 13, 2025
@picnixz picnixz added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Jan 13, 2025
@picnixz
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picnixz commented Jan 14, 2025 •
edited
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cc @iritkatriel @1st1 @gvanrossum as the authors of PEP-654.

@iritkatriel
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I think it's fine to add a frame in the traceback for the except Exception* line. It would be clearer than making it look like the raise Exception line created an ExceptionGroup..

@iritkatriel iritkatriel self-assigned this Jan 15, 2025
iritkatriel added a commit to iritkatriel/cpython that referenced this issue Jan 18, 2025
@iritkatriel
pythongh-128799: Add frame of except* to traceback when wrapping a na…
9716caa 
…ked exception
@iritkatriel iritkatriel linked a pull request Jan 18, 2025 that will close this issue
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@iritkatriel
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PR to fix it: #128971

I would call this a bug and backport. @gvanrossum do you agree?

@iritkatriel
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With the PR:

>>> def f():
...     try:
...         raise Exception  # Exception (inner) traceback terminates here
...     except* Exception as e:
...         raise
... 
... f()  # ExceptionGroup (wrapper) traceback terminates here
... 
  + Exception Group Traceback (most recent call last):
  |   File "<python-input-0>", line 7, in <module>
  |     f()  # ExceptionGroup (wrapper) traceback terminates here
  |     ~^^
  |   File "<python-input-0>", line 4, in f
  |     except* Exception as e:
  |         raise
  | ExceptionGroup:  (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "<python-input-0>", line 3, in f
    |     raise Exception  # Exception (inner) traceback terminates here
    |     ^^^^^^^^^^^^^^^
    | Exception
    +------------------------------------
>>>

@gvanrossum-ms
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Why does the raise on line 5 show up in the traceback?

@iritkatriel
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Why does the raise on line 5 show up in the traceback?

Good catch. It's because the location info for the except* in the code object spans the whole block. I'll need to fix that:

>>> def f():
...     try:
...         raise Exception  # Exception (inner) traceback terminates here
...     except* Exception as e:
...         1
...         2
...         3
...         raise
... 
... f()  # ExceptionGroup (wrapper) traceback terminates here
... 
  + Exception Group Traceback (most recent call last):
  |   File "<python-input-3>", line 10, in <module>
  |     f()  # ExceptionGroup (wrapper) traceback terminates here
  |     ~^^
  |   File "<python-input-3>", line 4, in f
  |     except* Exception as e:
  |     ...<3 lines>...
  |         raise
  | ExceptionGroup:  (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "<python-input-3>", line 3, in f
    |     raise Exception  # Exception (inner) traceback terminates here
    |     ^^^^^^^^^^^^^^^
    | Exception
    +------------------------------------

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gh-128799: Add frame of except* to traceback when wrapping a naked exception iritkatriel/cpython
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@jobh @iritkatriel @picnixz @gvanrossum-ms