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Feature Request: Add a generic property class #758
Description
Can you add a generic counterpart to the property decorator class to allow for proper typing of object properties. This would align with many other new classes in the typing library to improve type hinting in our code.
Here's my stab at an implementation:
from __future__ import annotations
from typing import Generic, TypeVar, Callable, Any, cast, get_type_hints, Optional, Type
T = TypeVar('T')
class Property(property, Generic[T]):
def __init__(self,
fget: Optional[Callable[[Any], T]] = None,
fset: Optional[Callable[[Any, T], None]] = None,
fdel: Optional[Callable[[Any], None]] = None,
doc: Optional[str] = None):
super().__init__(fget=fget, fset=fset, fdel=fdel, doc=doc)
def __get__(self, obj: Any, obj_type: type = None) -> T:
return cast(T, super().__get__(obj, obj_type))
def __set__(self, obj: Any, value: T) -> None:
super().__set__(obj, value)
def __delete__(self, obj: Any) -> None:
super().__delete__(obj)
def getter(self, fget: Callable[[Any], T]) -> Property[T]:
return cast(Property[T], super().getter(fget))
def setter(self, fset: Callable[[Any, T], None]) -> Property[T]:
return cast(Property[T], super().setter(fset))
def deleter(self, fdel: Callable[[Any], None]) -> Property[T]:
return cast(Property[T], super().deleter(fdel))This would allow us to do the following:
from typing import Property
IntProperty = Property[int]
class C:
def __init__(self, int_prop: int, str_prop: str):
self._int_prop = int_prop
self._str_prop = str_prop
@IntProperty
def int_prop(self) -> int:
return self._int_prop
@Property[str] # Relaxed syntax brought to you by PEP 614
def str_prop(self) -> str:
return self._str_prop
@str_prop.setter
def str_prop(self, new_str_prop: str) -> None:
self._str_prop = new_str_propI'm not sure if I have all the subtleties of the implementation correct, however my own cursory tests on python 3.9 work.
Activity
gvanrossum commentedon Sep 30, 2020
I'm not sure what this give beyond the standard property. Is there anything in your example program whose type gets inferred incorrectly in your example if you just use
@property? Or is there anything which mypy incorrectly disallows when using that?bubblefoil commentedon May 5, 2021
In my case, I have a generic class with a property of that generic type. I can type-hint the
@propertyannotated function and PyCharm would understand it. However, if I type-hint@foo.setterfunction, PyCharm has no idea what type the setter expects.jp-larose commentedon May 5, 2021
Sorry, I never came back to this until now...
The point for me was to ensure/validate that the getter and setter for a given property had a matching type.
srittau commentedon May 13, 2021
This honestly sounds like a bug/missing feature in PyCharm, although I think mypy has similar issues. The type of a property is the same as the return type of the getter function. (It's a bit more complicated if it has getter and setter methods accepting different types, but the principle is the same.) I don't think a generic would make any difference in the provided code samples.
srittau commentedon May 13, 2021
One thing we could improve is the types of
propertyin typeshed, though. Making it generic there sounds like a good idea to me.jp-larose commentedon May 18, 2021
I think this might be more appropriate than my initial suggestion.
srittau commentedon Nov 4, 2021
Closing this as a duplicate of #594 and python/typeshed#4731.