`warnings.filterwarnings` do not allow filtering warning that starts with empty line

[Czaki]

Bug report

Bug description:

# Add a code block here, if required

I have found that I cannot filter warnings that start with an empty line. Like this warning in pandas: https://github.com/pandas-dev/pandas/blob/488f6bd5c08111603549d42effc6dbb7fec935f0/pandas/__init__.py#L221-L231

On a project that I work (napari) we have a rule "error:::napari" in pytest that changes all warning into errors. If it is possible, then we solve warnings immediately, but some of them needs to be ignored (like linked one).

Based on documentation, I assume that rule "ignore:Pyarrow will become a required dependency of pandas in the next major release of pandas.*:DeprecationWarning", should work. But it don't.

Rule is effectively calling warnings.filterwarnings("ignore", "Pyarrow will become a required dependency of pandas in the next major release of pandas.*", DeprecationWarning).

I have found that in filterwarnings the message is changed to regexp using only re.IGNORECASE flag

cpython/Lib/warnings.py

Line 160 in d1b031c

message = re.compile(message, re.I)

without re.MULTILINE flag.

And then the match of the regexp is used

cpython/Lib/warnings.py

Line 375 in d1b031c

if ((msg is None or msg.match(text)) and

.

Based on my knowledge, the problem could be solved by either adding re.MULTILINE to compile flags or using msg.search during filtering.

Which solution is preferred? I could make a PR if someone point which one I should select.

CPython versions tested on:

3.11

Operating systems tested on:

Linux

[serhiy-storchaka]

Try to use
"\nPyarrow will become a required dependency of pandas"
or
"(?s:.*)Pyarrow will become a required dependency of pandas"

[Czaki]

First do not work,
Second fail, because pytest uses : as separtor but I found that "ignore:(?s).*Pyarrow will become a required dependency of pandas:DeprecationWarning" works.

So maybe I should think about possible documentation update?

[antonymilne]

The following works for me - note the \\:

\\nPyarrow will become a required dependency of pandas

For anyone coming from the pandas issue, in this case the (?s).* solution is better than \\n since it will also work when pandas-dev/pandas#57003 gets released.

Thanks for figuring all this out @Czaki - I was faced with the same difficulty and your work on it is very helpful!