The warnings module tries to find and show the line of code which is responsible for a warning, for the same reasons that tracebacks show a line of code from each stack frame. However, they work in quite different ways.

Native tracebacks, the traceback module and pdb all do something like this:

frame.f_code.co_filename

But warnings does something like this (paraphrasing C code in _warnings.c):

frame.f_globals.get('__file__', sys.argv[0])

This causes problems for interactive interpreters like IPython, because there are multiple pieces of entered code which have to share the same global namespace. E.g. https://github.com/ipython/ipython/issues/11080

This was raised a long time ago in #1692664. Back then, the answer was that co_filename could be wrong if the path of a pyc file changed. However, that issue was fixed in #1180193. And it seems that the co_filename approach must largely work today, because tracebacks and pdb rely on it.

So I'm proposing to make warnings match how those other tools find filenames. I think this should also be a minor simplification of the code.

Hi Brett! If you get a moment, any review of the linked PR would be welcome. :-)

https://github.com/python/cpython/pull/6622

I just added myself to review the PR. At worst I will get to it during the
sprints at PyCon US.

On Thu, 3 May 2018 at 12:52 Thomas Kluyver <report@bugs.python.org> wrote:

>
> Thomas Kluyver <thomas@kluyver.me.uk> added the comment:
>
> Hi Brett! If you get a moment, any review of the linked PR would be
> welcome. :-)
>
> https://github.com/python/cpython/pull/6622
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue33375>
> _______________________________________
>

Thanks! No rush, I just thought I'd take the opportunity when you added yourself to the nosy list.

New changeset 11a896652ee98aa44e59ed25237f9efb56635dcf by Brett Cannon (Thomas Kluyver) in branch 'master':
bpo-33375: Get filename for warnings from frame.f_code.co_filename (GH-6622)
https://github.com/python/cpython/commit/11a896652ee98aa44e59ed25237f9efb56635dcf

New changeset 3f45f5da8eb052f1b54d37086c67b7094f35b67b by Brett Cannon (Zackery Spytz) in branch 'master':
bpo-33375: Fix GCC warning in Python/_warnings.c (GH-7556)
https://github.com/python/cpython/commit/3f45f5da8eb052f1b54d37086c67b7094f35b67b

Thanks everyone for making this happen!

This change introduced a regression: bpo-33912 "[EASY] test_warnings: test_exec_filename() fails when run with -Werror". I consider that it's an easy issue, please don't fix it, but explain how to fix it and let a newcomer to fix it!
https://mail.python.org/pipermail/python-committers/2017-June/004564.html

I closed bpo-8787 as a duplicate of this issue.