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property.__get__: overload to model class-access behavior #13769

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Apr 2, 2025
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property.__get__: overload to model class-access behavior #13769

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3 changes: 3 additions & 0 deletions stdlib/builtins.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -1281,6 +1281,9 @@ class property:
def getter(self, fget: Callable[[Any], Any], /) -> property: ...
def setter(self, fset: Callable[[Any, Any], None], /) -> property: ...
def deleter(self, fdel: Callable[[Any], None], /) -> property: ...
@overload
def __get__(self, instance: None, owner: type, /) -> Self: ...
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The second argument is optional and unused, we should reflect that:

Suggested change
def __get__(self, instance: None, owner: type, /) -> Self: ...
def __get__(self, instance: None, owner: Unused = None, /) -> Self: ...

(Unused might need to be imported from _typeshed. Also, we might need to silence mypy's overlap warning.)

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@AlexWaygood AlexWaygood Apr 2, 2025 •
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The second argument is optional

I don't think that's true? If None is supplied for the first parameter, it's invalid to leave the second parameter unspecified:

>>> class Foo:
...     @property
...     def bar(self): return 42
...     
>>> b = Foo.__dict__["bar"]
>>> b.__get__(None)
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    b.__get__(None)
    ~~~~~~~~~^^^^^^
TypeError: __get__(None, None) is invalid

Only if I provide the second paramter is it valid to give None for the first parameter:

>>> b.__get__(None, Foo)
<property object at 0x102f3c860>

It's true that if you provide an instance as the first parameter, rather than None, then it's okay to omit the second parameter:

>>> b.__get__(Foo())
42

but that's already covered by the second overload immediately below?

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@srittau srittau Apr 2, 2025 •
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I don't think that's true? If None is supplied for the first parameter, it's invalid to leave the second parameter unspecified:

Which means that both the linked pseudo code is wrong and that the C implementation contradicts the specification, but it is what it is ...

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Yeah, I think @sharkdp would be the first to tell you that we've been having "a lot of fun" recently at red-knot trying to figure out which bits of documentation, specification and pseudo-code regarding the descriptor protocol are actually precise descriptions of the real runtime behaviour, and which are just approximations 😆

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Which means that both the linked pseudo code is wrong and that the C implementation contradicts the specification, but it is what it is ...

Right. There is some precedent for modeling __get__ this way in typeshed. For example, see types.FunctionType or builtins.classmethod:

typeshed/stdlib/types.pyi

Lines 111 to 114 in 977f300

@overload
def __get__(self, instance: None, owner: type, /) -> FunctionType: ...
@overload
def __get__(self, instance: object, owner: type | None = None, /) -> MethodType: ...

typeshed/stdlib/builtins.pyi

Lines 160 to 163 in 977f300

@overload
def __get__(self, instance: _T, owner: type[_T] | None = None, /) -> Callable[_P, _R_co]: ...
@overload
def __get__(self, instance: None, owner: type[_T], /) -> Callable[_P, _R_co]: ...

For the property descriptor, it's not particularly important, because the owner attribute is not used. But for some descriptors, it is essential that owner is passed in when instance = None. For example, classmethod.__get__ needs the owner argument to create a <bound method Owner.some_class_method of <class 'Owner'>.

@overload
def __get__(self, instance: Any, owner: type | None = None, /) -> Any: ...
def __set__(self, instance: Any, value: Any, /) -> None: ...
def __delete__(self, instance: Any, /) -> None: ...
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