"error: only instance methods can be decorated with @property" is incorrect for Python 3.9 and later

[zackw]

Bug Report

The code shown in the "to reproduce" section works correctly in Python 3.10 and later (and would work correctly in 3.9 and later if it didn't use inspect.get_annotations), but triggers an unconditional error message from mypy: "Only instance methods can be decorated with @property".

As far as I can tell, that was true in 3.8 and earlier, but that restriction was relaxed as a new feature of 3.9. See the "Changed in 3.9" note at the end of https://docs.python.org/3.9/library/functions.html#classmethod, which specifically calls out @property as something that can now be wrapped in @classmethod.

This is closely related to, but not quite the same as, #2563.

To Reproduce

from inspect import get_annotations

class BugDemo:
    @classmethod
    @property
    def permitted_keys(cls) -> frozenset[str]:
        return frozenset(get_annotations(cls).keys())

    a: str
    b: int

if __name__ == "__main__":
    print(repr(BugDemo.permitted_keys))

playground: https://mypy-play.net/?mypy=latest&python=3.10&gist=91eef6c76933a64774b3ac310976da27

Expected Behavior

This program should be considered type-correct.

Actual Behavior

$ python3.10 bug.py
frozenset({'b', 'a'})

$ mypy bug.py
bug.py:4: error: Only instance methods can be decorated with @property  [misc]

$ mypy --python-version=3.10 bug.py
bug.py:4: error: Only instance methods can be decorated with @property  [misc]

Your Environment

• Mypy version used: 1.18.2 (compiled: yes)
• Mypy command-line flags: see "actual behavior" section
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[sterliakov]

Allowing @classmethod @property was a huge mistake, only present in 3.10 (3.9 is already EOL) - this stacking was deprecated in 3.11 and removed in 3.13. deprecation docs (first bullet), removal docs.

I think it's OK for mypy to unconditionally reject code that is only supported on 3.10, especially given that this behavior was considered so problematic that it was worth an immediate revert in the next major release.

[zackw]

Ugh. I was looking at the documentation for 3.11 only (my lowest actual supported Python version is 3.11); the fact that this feature was backed out was not nearly as obvious as it should have been in the 3.11 docs (it is clear from the 3.13 documentation).

I'd suggest that the mypy error message should be revised to read "Chaining @classmethod with @property was deprecated in 3.11 and removed in 3.13, see <https://github.com/python/cpython/issues/89519>." This would make it clearer that it isn't a case of mypy being out of step with cpython.