[New-bugs-announce] [issue43862] warnings: -W option and PYTHONWARNINGS now use the message as a regex

[STINNER Victor]

New submission from STINNER Victor <vstinner at python.org>:

I propose to change the -W command line option and the PYTHONWARNINGS environment variable to use the message as a regular expression in Python 3.10. Or does anyone have a reason to keep the current behavior as it is?

--

Python provides two ways to specify warnings filters:

* -W command line option: can be used multiple times
* PYTHONWARNINGS environment variable: can contain multiple options separated by commas

While the Python API warnings.filterwarnings(action, message="", ...) uses the message as a regular expression, -W and PYTHONWARNINGS require to match *exactly* the *whole* message.

For example, if you only want to ignore the new distutils deprecation warning, you must write exactly:

$ ./python -X dev -W 'ignore:The distutils package is deprecated and slated for removal in Python 3.12. Use setuptools or check PEP 632 for potential alternatives:DeprecationWarning' -c 'import distutils'

I use -X dev to show DeprecationWarning, or you can also use -Wdefault if you prefer.

If the deprecation warning changes in Python or if you have a single typo, the warning is not ignored. Example with a typo ("3.13" rather than "3.12"):

$ ./python -X dev -W 'ignore:The distutils package is deprecated and slated for removal in Python 3.13. Use setuptools or check PEP 632 for potential alternatives:DeprecationWarning' -c 'import distutils'
<string>:1: DeprecationWarning: The distutils package is deprecated and slated for removal in Python 3.12. Use setuptools or check PEP 632 for potential alternatives

The PYTHONWARNINGS has another limitation: you cannot specify a message if it contains a comma (","). Hopefully, Python doesn't raise warnings containing comma, right? Well... Just one example:

Lib/distutils/sysconfig.py:554:        warnings.warn('SO is deprecated, use EXT_SUFFIX', DeprecationWarning, 2)

You cannot only ignore the message:

$ PYTHONWARNINGS='ignore:SO is deprecated, use EXT_SUFFIX:DeprecationWarning' ./python -c 'import sys; print(sys.warnoptions); print(len(sys.warnoptions))'
Invalid -W option ignored: invalid action: 'use EXT_SUFFIX'
['ignore:SO is deprecated', ' use EXT_SUFFIX:DeprecationWarning']
2

You can only try to use "module" and "lineno" parameters of a warning filter, which are more fragile and hard to use to use.

----------
components: Library (Lib)
messages: 391190
nosy: vstinner
priority: normal
severity: normal
status: open
title: warnings: -W option and PYTHONWARNINGS now use the message as a regex
versions: Python 3.10

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43862>
_______________________________________