Class inheriting from property doesn't typecheck properly when used as a decorator

[matthchr]

I have a custom class inheriting from property. When I go to use this as a decorator though mypy seems to miss that it's a property at all. This is with Python 3.6.3 and mypy 0.650.

Example:

class CustomProperty(property):
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        super().__init__(fget, fset, fdel, doc)

class A:
    def __init__(self) -> None:
        self._val = 1

    @CustomProperty
    def val(self) -> int:
        return self._val

    @val.setter
    def val(self, v: int) -> None:
        self._val = v


def test_a() -> None:
    print(A.val)

Running mypy on this gives the following output:
mypy minimal2.py

minimal.py:14: error: Name 'val' already defined on line 10
minimal.py:14: error: Name 'val' is not defined
minimal.py:20: error: "Type[A]" has no attribute "val"

Contrast this with the same code with the builtin property which typechecks fine:

class B:
    def __init__(self) -> None:
        self._val = 1

    @property
    def val(self) -> int:
        return self._val

    @val.setter
    def val(self, v: int) -> None:
        self._val = v

def test_b() -> None:
    print(B.val)

It's possible that this is related to #1529 where @ilevkivskyi suggested that:

The original example now passes mypy, not sure however how useful the subclass can be (property is heavily special-cased in mypy), but at least the original bug is fixed, so closing this.

Is subclassing property frowned upon (or just not supported in mypy?).

[msullivan]

Unfortunately mypy doesn't support subclassing property right now.

In simple use-cases, you can get away with something like

if TYPE_CHECKING:
    CustomProperty = property
else:
    # real CustomProperty definition

but this won't work if you need a custom constructor

[ilevkivskyi]

In some sense this is a "duplicate" of #220, but the latter is too broad, so I propose to keep this open instead of accumulating example of unsupported patterns in that issue.

[johhnry]

Hi! Are there any update on this? I am using MyPy 1.8.0 and this doesn't work:

class custom(property):
    pass

class A:
    @custom
    def p(self):
        return 0
        
    @custom.setter
    def p(self, v: int) -> None:
        return

main.py:9: error: Name "p" already defined on line 5  [no-redef]
Found 1 error in 1 file (checked 1 source file)

The fix from @msullivan works thought but it's not clean...

Thanks!