Make classproperty descriptor work

[JukkaL]

As discussed in #2266, this apparently doesn't work yet:

# See http://stackoverflow.com/questions/3203286/how-to-create-a-read-only-class-property-in-python

from typing import *

T = TypeVar('T')
V = TypeVar('V')

class classproperty(Generic[T, V]):

    def __init__(self, getter: Callable[[Type[T]], V]) -> None:
        self.getter = getter

    def __get__(self, instance: Any, owner: Type[T]) -> V:
        # instance is really None, but we don't care
        return self.getter(owner)

class C:
    @classproperty    # <-- error here
    def foo(cls) -> int:
        return 42

reveal_type(C.foo)

@gvanrossum reported the following output from mypy:

classprop.py: note: In class "C":
classprop.py:20: error: Argument 1 to "classproperty" has incompatible type Callable[[C], int]; expected Callable[[Type[None]], int]
classprop.py: note: At top level:
classprop.py:25: error: Revealed type is 'classprop.classproperty[builtins.None, builtins.int*]'

[tyrion]

It seems to work in this way:

from typing import *

T = TypeVar("T")
V = TypeVar("V")

class classproperty(Generic[T, V]):
    def __init__(self, getter: Callable[[Type[T]], V]) -> None:
        self.getter = getattr(getter, "__func__", getter)

    def __get__(self, instance: Any, owner: Type[T]) -> V:
        return self.getter(owner)

class C:
    @classproperty
    @classmethod
    def foo(cls) -> int:
        return 42

reveal_type(C.foo)

I don't think it is ideal to have to use both classproperty and classmethod but I couldn't find an other way to make it work.

[oakkitten]

the last example doesn't work with TypeVars:

from typing import *

T = TypeVar("T")
V = TypeVar("V")

class classproperty(Generic[T, V]):
    def __init__(self, getter: Callable[[Type[T]], V]) -> None:
        self.getter = getattr(getter, "__func__", getter)

    def __get__(self, instance: Any, owner: Type[T]) -> V:
        return self.getter(owner)

F = TypeVar("F", bound='Foo')

class Foo:
    @classproperty
    @classmethod
    def foo(cls: Type[F]) -> F:
        return cls()

class Bar(Foo):
    pass

reveal_type(Bar.foo)

yields

main.py:24: error: Argument 2 to "__get__" of "classproperty" has incompatible type "Type[Bar]"; expected "Type[F]"
main.py:24: note: Revealed type is 'F`-1'
Found 1 error in 1 file (checked 1 source file)

seems to work if you remove T, although it feels like a wrong thing to do?

[sirosen]

Under 3.9, the docs are now recommending the use of classmethod(property(...)) to handle class properties. This is reinforced by sphinx having special-cased support for this case -- if you want autodoc to produce class property docs, it must be done by wrapping a property in the classmethod descriptor.

If you try to do this and run mypy, however, the class property type is deduced as () -> T instead of T.

e.g.

class A:
    @classmethod
    @property
    def foo(cls) -> int:
        return 1

reveal_type(A.foo)

shows Revealed type is "def () -> builtins.int"

Would it be possible to special-case classmethod(property(...)) to make that case behave correctly? That would satisfy at least some cases and harmonize with 3.9+ docs and sphinx behavior.

Right now, trying to declare a read-only class property which satisfies the trifecta of

• runtime behavior
• sphinx autodoc extension
• mypy

is quite tricky.

[JacobHayes]

I've been using a TypeVar and wrapping func to hack this together:

from collections.abc import Callable
from typing import TypeVar

PropReturn = TypeVar("PropReturn")


def classproperty(meth: Callable[..., PropReturn]) -> PropReturn:
    """Access a @classmethod like a @property."""
    # mypy doesn't understand class properties yet: https://github.com/python/mypy/issues/2563
    return classmethod(property(meth))  # type: ignore


class A:
    @classproperty
    def x(cls) -> int:
        return 5


print(A.x)  # 5
reveal_type(A.x)  # note: Revealed type is "builtins.int"
print(A().x)  # 5
reveal_type(A().x)  # note: Revealed type is "builtins.int"

Rather than muck with casts (since they'd be lying anyway 😁), the # type: ignore inside classproperty works around Argument 1 to "classmethod" has incompatible type "property"; expected "Callable[..., Any]" and Incompatible return value type.