3.13 Regression: `inspect.getsource()` returns incorrect source code

[mhils]

Bug report

Bug description:

In Python 3.13rc1, inspect.getsource() sometimes returns a "random" line for some objects.
For example, here's ssl.AlertDescription:

$ docker run --rm python:3.13-rc python -c "import inspect, ssl; print(inspect.getsource(ssl.AlertDescription))"
        super().shutdown(how)

This line is likely https://github.com/python/cpython/blob/v3.13.0rc1/Lib/ssl.py#L1334.

In contrast, on 3.12 and below, this raises an OSError (not ideal but also not wrong):

$ docker run --rm python:3.12-rc python -c "import inspect, ssl; print(inspect.getsource(ssl.AlertDescription))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/lib/python3.12/inspect.py", line 1285, in getsource
    lines, lnum = getsourcelines(object)
                  ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/inspect.py", line 1267, in getsourcelines
    lines, lnum = findsource(object)
                  ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/inspect.py", line 1112, in findsource
    raise OSError('could not find class definition')
OSError: could not find class definition

I'm not sure why this is happening, but it also affects pytest.PytestAssertRewriteWarning and pluggy.PluggyTeardownRaisedWarning, which both extend custom classes that override __module__.

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12, 3.13

Operating systems tested on:

Linux

Linked PRs

• gh-122981: Fix inspect.getsource() for generated classes with Python base classes #123001
• [3.13] gh-122981: Fix inspect.getsource() for generated classes with Python base classes (GH-123001) #123182

[devdanzin]

Bisected to 153b3f7, introduced by #118475.

It happens because class ssl.AlertDescription now has a __firstlineno__ attribute, and that is erroneously interpreted as a line number in ssl.py. It shares the same line number (1334) with other classes, like SSLErrorNumber, created via IntEnum._convert_. This line number corresponds to IntEnum's line number in enum.py instead.

So ISTM the issue is that the construction of __firstlineno__ for these classes is capturing the line number of the class generator definition instead of the class creation site.

cc @serhiy-storchaka

[serhiy-storchaka]

Thank you for your report @mhils. I agree with @devdanzin analysis. __firstlineno__ is not set for these enum classes, but inherited from the base class. There is a test for generated class, but it does not have a base class that has __firstlineno__.

#123001 should fix this.

[mhils]

Awesome - thank you two for getting to the bottom of this with lightning speed! ⚡🍰 😃