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Title: warnings.catch_warnings is not thread-safe
Type: behavior Stage:
Components: Documentation, Library (Lib) Versions: Python 3.1, Python 3.2, Python 2.7, Python 2.6
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: georg.brandl Nosy List: akuchling, georg.brandl, ggenellina, pitrou
Priority: normal Keywords: patch

Created on 2009-08-05 04:52 by ggenellina, last changed 2022-04-11 14:56 by admin. This issue is now closed.

File name Uploaded Description Edit ggenellina, 2009-08-05 04:52 demo code showing the problem
warnings.diff ggenellina, 2009-11-15 11:15
Messages (4)
msg91301 - (view) Author: Gabriel Genellina (ggenellina) Date: 2009-08-05 04:52
warnings.catch_warnings is a context manager supposed to save and 
restore warnings filters, and optionally record all warnings issued.

But it does so in a completely thread-unsafe way, by replacing the 
module's "showwarning" and "filters" attributes on enter, and restoring 
them on exit. If the __enter__ / __exit__ calls of two threads overlap, 
after leaving the last block the warnings state is not the same as the 
original state, as it should be.

I don't know how to fix this, other than using locks (that could block 
indefinitely) or severely restricting how catch_warnings may be used. 
At least, this issue should be documented.
msg91318 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2009-08-05 12:50
Any function whose purpose is to mutate global (non thread-local) state
is inherently "thread-unsafe". The only reasonable possibility is to
document it as such.
msg95285 - (view) Author: Gabriel Genellina (ggenellina) Date: 2009-11-15 11:15
Document that warnings.catch_warnings is not thread safe.
msg102185 - (view) Author: A.M. Kuchling (akuchling) * (Python committer) Date: 2010-04-02 17:54
Added a warning to 2.x trunk in r79607, and Gabriel's doc change in r79608.  This thread-unsafety seems specialized and rarely of great importance, so I used the note:: directive, not warning::.
Date User Action Args
2022-04-11 14:56:51adminsetgithub: 50896
2010-04-02 17:54:34akuchlingsetstatus: open -> closed

nosy: + akuchling
messages: + msg102185

resolution: fixed
2009-11-15 11:15:30ggenellinasetfiles: + warnings.diff
keywords: + patch
messages: + msg95285
2009-08-05 12:50:46pitrousetnosy: + georg.brandl, pitrou
messages: + msg91318

assignee: georg.brandl
components: + Documentation
2009-08-05 04:52:59ggenellinacreate