Regular expressions in -W and PYTHONWARNINGS

Core Development
1

It was an old wart that while filterwarnings() and simplefilter() accepted regular expressions for module name and warning message, the CLI -W option and the PYTHONWARNINGS environment variable only allowed to specify literal strings. For a long time, there was a discrepancy between behavior and documentation which was resolved by changing the documentation.

But from now, you could specify a regular expression surrounded by slashes (for example, -W 'ignore:/.*was never awaited$/:RuntimeWarning:/.*\.util/'). Technically, this is a breaking change, but I hope that very few warning messages start with a slash. If you have any suggestions or objections to the idea, implementation or documentation, you are welcome. Anyway, I hope this information is useful to you.

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2

Why do we even need the slashes?

3

If we drop the slashes and unconditionally interpret literals as regexes then something like

PYTHONWARNINGS='ignore:setDaemon() is deprecated:DeprecationWarning:'

would stop working because () doesn’t match regex pattern ()?

3 Likes
4

I like the / slashes approach. It is sufficiently unique it won’t conflict with existing use cases.

1 Like