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gh-118465: Add __firstlineno__ attribute to class#118475

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serhiy-storchaka merged 7 commits into
python:mainfrom
serhiy-storchaka:class-firstlineno
May 6, 2024
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gh-118465: Add __firstlineno__ attribute to class#118475
serhiy-storchaka merged 7 commits into
python:mainfrom
serhiy-storchaka:class-firstlineno

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@serhiy-storchaka

@serhiy-storchaka serhiy-storchaka commented May 1, 2024 •
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It is set by compiler with the line number of the first line of the class definition.

📚 Documentation preview 📚: https://cpython-previews--118475.org.readthedocs.build/

@serhiy-storchaka
pythongh-118465: Add __firstlineno__ attribute to class …
2d78936 
It is set by compiler with the line number of the first line of
the class definition.
@bedevere-app bedevere-app Bot mentioned this pull request May 1, 2024
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@serhiy-storchaka @markshannon
Update Doc/reference/datamodel.rst …
2ac81ad 
Co-authored-by: Mark Shannon <mark@hotpy.org>
@JelleZijlstra

JelleZijlstra commented May 4, 2024

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It is possible to construct classes without __firstlineno__:

def f():
    __firstlineno__ = 1
    class X:
        nonlocal __firstlineno__
    return X
print(f().__firstlineno__)  # AttributeError

Or:

class Y:
    global __firstlineno__
print(Y.__firstlineno__)  # AttributeError

This is obviously an extreme edge case though. You could also do this by manipulating locals() in the class body or the metaclass.

Is it worth it to make inspect smarter in the presence of classes without __firstlineno__?

  File "/Users/jelle/py/cpython/insp.py", line 18, in <module>
    print(inspect.findsource(X))
          ~~~~~~~~~~~~~~~~~~^^^
  File "/Users/jelle/py/cpython/Lib/inspect.py", line 1070, in findsource
    return lines, object.__firstlineno__ - 1
                  ^^^^^^^^^^^^^^^^^^^^^^
AttributeError: type object 'X' has no attribute '__firstlineno__'

@serhiy-storchaka

serhiy-storchaka commented May 5, 2024

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Good point. I thought that it is the same as for __module__ and __qualname__, but it turned out that they are set implicitly in the type constructor even when not specified in the class dict. They are set even for type('A', (), {}).

We can try to set __firstlineno__ to the current line number in the top Python frame, but it is safer to not set it at all. Setting __firstlineno__ for classes created by the type() call rather than in the class statement does not help in the only use case for this attribute.

I think that for backward compatibility getsource() should convert an AttributeError into OSError which was raised before for classes created by the type() call.

@serhiy-storchaka serhiy-storchaka merged commit 153b3f7 into python:main May 6, 2024
@serhiy-storchaka serhiy-storchaka deleted the class-firstlineno branch May 6, 2024 09:02
SonicField pushed a commit to SonicField/cpython that referenced this pull request May 8, 2024
@serhiy-storchaka @SonicField
pythongh-118465: Add __firstlineno__ attribute to class (pythonGH-118475 …
7670241 
)

It is set by compiler with the line number of the first line of
the class definition.
@frenzymadness frenzymadness mentioned this pull request May 13, 2024
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@devdanzin devdanzin mentioned this pull request Aug 14, 2024
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