Annotating custom constructor methods for generic types. #2025
-
|
How to annotate # fmt: off
from collections.abc import Mapping
from typing import overload
dict(bar=3) # SHOULD PASS (pyright: ✅, mypy: ✅)
dict[str, int](bar=3) # SHOULD PASS (pyright: ✅, mypy: ✅)
dict[int, int](bar=3) # SHOULD ERROR (pyright: ✅, mypy: ❌)
class Foo[K, V](Mapping[K, V]):
@overload
@classmethod
def new(cls, items: Mapping[K, V], /) -> "Foo[K, V]": ...
@overload
@classmethod
def new(cls, items: Mapping[str, V] = ..., /, **kwargs: V) -> "Foo[str, V]": ...
Foo.new(bar=3) # SHOULD PASS (pyright: ✅, mypy: ✅)
Foo[str, int].new(bar=3) # SHOULD PASS (pyright: ✅, mypy: ✅)
Foo[int, int].new(bar=3) # SHOULD ERROR (pyright: ❌, mypy: ❌)My analysis:
|
Beta Was this translation helpful? Give feedback.
Replies: 1 comment 6 replies
-
|
I think option 1 is the right approach, but you need to use |
Beta Was this translation helpful? Give feedback.
-
|
Like so? Code sample in pyright playground This does seem to work with _m0 = Foo.new(bar=3)
reveal_type(_m0) # "Foo[str, Unknown]"Any idea why it fails to bind |
Beta Was this translation helpful? Give feedback.
-
Yes, |
Beta Was this translation helpful? Give feedback.
-
|
Just to make sure that I get this right, when you say:
What exactly counts as *a constructor*? Only the standard constructor Well, actually, ... it does seem to produce the desired inference if we define I guess this makes sense insofar as that if we were to access, say, some generic class-attributes inside So if we want full type-checking for a custom constructor, one either should use a function defined outside the class, a Learned something new today, thanks as always for your insights. |
Beta Was this translation helpful? Give feedback.
-
|
Though I have to say, having to define metaclasses makes this whole thing quite un-ergonomic. In fact, there seems to be another type-checker divergence here: Code sample in pyright playground, https://mypy-play.net/?mypy=latest&python=3.13&gist=d902e42012932c4a8892c75c46d170d4 from typing import Self
class Vector[T]:
@classmethod
def from_list(cls, x: list[T]) -> Self: ...
# shows mypy: Vector[str], pyright: Vector[Unknown]
reveal_type(Vector.from_list(["abc"])) |
Beta Was this translation helpful? Give feedback.
-
|
Would you consider |
Beta Was this translation helpful? Give feedback.
-
The typing specs IMO are silent rather than clear on this. The only examples of omitting generics I see are
In both of these examples, the omission resulting in an implicit parameterisation by |
Beta Was this translation helpful? Give feedback.
Yes,
Foo.newbindsFoo[Any, Any]to thenewmethod. The types of class-scoped type variables are inferred from argument types only when you call a constructor. This doesn't work for other methods. In other cases, the specialization comes from the bound class — in this case,FooorFoo[Any, Any]since you haven't specified any default values for the type variables.