Using `x = property(get_x)` in a class definition misbehaves

[finite-state-machine]

[It's difficult to believe this hasn't been reported before, but, with apologies, I haven't been able to find an open issue describing this.]

Bug Report

Mypy doesn't support using @property as a function in a class definition. This makes it challenging to work around #6700.

We might expect these declarations to be similar (a11y version follows):

# with '@property' used as a function:            │  # with '@property' used as a decorator:
                                                  │
class SomeClass:                                  │  class SomeClass:
                                                  │
    def _get_foo(self) -> int:                    │      def _get_foo(self) -> int:
        return 42                                 │          return 42
                                                  │
    foo = property(get_foo)                       │      @property
                                                  │      def foo(self) -> int:
                                                  │          return _get_foo()

Accessible version of the above

# with '@property' used as a function:

class SomeClass:

    def _get_foo(self) -> int:
        return 42

    foo = property(get_foo)


# with '@property' used as a decorator:

class SomeClass:

    def _get_foo(self) -> int:
        return 42

    @property
    def foo(self) -> int:
        return _get_foo()

But in mypy, the left form results in foo having type Any, whether accessed on the class itself or an instance.

To Reproduce

mypy-play.net: gist

from typing_extensions import (
    assert_type,
    )

class SomeClass:
    @property
    def controlcase(self) -> int:
        return 42

    def get_testcase(self) -> int:
        return 42

    testcase = property(get_testcase)

inst = SomeClass()

# 'testcase' should behave the same way as 'controlcase':
reveal_type(SomeClass.controlcase)  # ... "def (self: SomeClass) -> int"
reveal_type(inst.controlcase)       # ... "int"

# but it does not:
reveal_type(SomeClass.testcase)     # ... "Any"
reveal_type(inst.testcase)          # ... "Any"

Expected Behavior

controlcase and testcase should be indistinguishable

Actual Behavior

Mypy doesn't understand @property when used as a function.

Your Environment

• Mypy version used: 1.8.0 (and probably at least as far as 0.730)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8, 3.12

[finite-state-machine]

Accessible version of the first code block in this issue's description:

# with '@property' used as a function:

class SomeClass:

    def _get_foo(self) -> int:
        return 42

    foo = property(get_foo)


# with '@property' used as a decorator:

class SomeClass:

    def _get_foo(self) -> int:
        return 42

    @property
    def foo(self) -> int:
        return _get_foo()

[erictraut]

The property decorator requires significant special-casing within a static type checker, so mypy's behavior here doesn't surprise me. You're using property in a very unusual manner here. FWIW, pyright's behavior is the same as mypy in this case.

[finite-state-machine]

You're using property in a very unusual manner here.

Agreed. I was hoping to work around #6700 as follows:

class SomeClass:
    ...
    some_prop = property(getter, setter)
    some_alias = property(getter, setter)

I guess I'll have to "spell it out" until #6700 is definitively addressed.