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warnings.warn's skip_file_prefixes doesn't work when provided __file__. #126209
Description
Bug report
Bug description:
I was attempting to make warning.warn skip all frames in the calling file by setting skip_file_prefixes=(__file__,). Frustratingly it wasn't working as expected and was blaming an intermediate frame in the file I was trying to skip. Looking at Lib/warnings.py I was confused as to why it wasn't working. After some digging I discovered there is a parallel C implementation in Python/_warnings.c.
The following script demonstrates the difference between the two implementations:
import sys
# Uncomment to compare the C and Py implementation of warnings.warn.
# sys.modules["_warnings"] = None
import warnings
# Desired input which works as expected with the Py impl, but not the C impl.
skip_file_prefixes=(__file__,)
# Hack to make the C implementation match the Py behavior:
# skip_file_prefixes=(__file__[:-1],)
def ccc():
warnings.warn("Hello", UserWarning, skip_file_prefixes=skip_file_prefixes)
def bbb():
ccc()
def aaa():
bbb()
aaa()Broken C implementation:
$ python sample.py
/tmp/sample.py:12: UserWarning: Hello
ccc()
Correct Python implementation:
$ python sample.py
sys:1: UserWarning: Hello
There is a slight implementation difference between Python/_warnings.c:is_filename_to_skip and Lib/warnings.py:_is_filename_to_skip. The Python implementation is uses filename.startswith(prefix) 1 while the C implementation uses PyUnicode_Tailmatch(filename, prefix, 0, -1, -1) 2. It looks like the C implementation should be something like PyUnicode_Tailmatch(filename, prefix, 0, PY_SSIZE_T_MAX, -1).
$ python --version
Python 3.12.7
CPython versions tested on:
3.12
Operating systems tested on:
Linux
Activity
zsaladin commentedon Nov 2, 2024
I will take a look
corona10 commentedon Nov 2, 2024
@zsaladin is a first-time contributor who is participating in the PyCon KR sprint today.
I will help him contribute to this issue :)
pythongh-126209: Fix the inconsistent results of c and python impleme…
pythongh-126209: Fix the inconsistent results of c and python impleme…
pythongh-126209: Add assert for length of warn result
pythongh-126209: Add a test for inconsistent behaviour of `skip_file_…
gh-126209: Fix inconsistency of `skip_file_prefixes` in `warnings.war…
pythongh-126209: Fix inconsistency of `skip_file_prefixes` in `warnin…
hugovk commentedon Jan 7, 2025
Thank you @nairb774 for reporting, @zsaladin for the PR, and @corona10 for helping :)
pythongh-126209: Fix inconsistency of `skip_file_prefixes` in `warnin…