Document or fix that `message` and `module` components of warnings filters have different meanings in -W and pytest filterwarnings

[mvashishtha]

Hi, I'm trying to raise an error when I get a string like

"Only 10000 rows were downloaded{extra_stuff_not_in_actual_string}client_row_transfer_limit{more_stuff_in_string}"

and I just want to raise an error for UserWarnings with messages containing the substring "client_row_transfer_limit".

This is easy to do with a regex matching the start of the string with the filterwarnings ini option:

#pytest.ini
[pytest]
filterwarnings =
	error:.*client_row_transfer_limit:UserWarning

I'm using pants pytest to run my test, and pants doesn't support the filterwarnings option, but it does support passing command-line args to pytest, so I passed "-W error:.*client_row_transfer_limit.*:UserWarning", which seemed to match the warning filter string syntax here that pytest links to. I was very confused to see that pytest wasn't raising any errors. After some digging, I found that the message and module components of the warnings filter are interpreted differently for the python argument -W (outside of the pytest context), and that

message is a literal string that the start of the warning message must contain (case-insensitively), ignoring any whitespace at the start or end of message

I tried "-W error:Only:UserWarning" and that finally matched the warning. I have two complaints:

• pytest documentation should warn that when using -W instead of filterwarnings, the message and module params are interpreted differently

• Is there any reason that pytest -W has to follow the convention of the generic python command-line option -W? Message pattern matching for -W is not as powerful. In this example, I only care about the client_row_transfer_limit part which appears at the beginning of my message.

• a detailed description of the bug or problem you are having

• output of pip list from the virtual environment you are using

• pytest and operating system versions

• minimal example if possible

[nicoddemus]

Hi,

pytest documentation should warn that when using -W instead of filterwarnings, the message and module params are interpreted differently

Agreed, would love to review/merge a PR in that direction. 👍

Is there any reason that pytest -W has to follow the convention of the generic python command-line option -W?

We decided to follow what Python does when supporting this feature, which sounds reasonable. Having a different behavior might be more surprising for users accustomed to the way the builtin feature works.