classmethod's in generic protocols with self-types #5872
Comments
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I think this is actually a duplicate of #2511. Here is a repro that doesn't include protocols and dataclasses: T = TypeVar('T', bound=B)
class B:
@classmethod
def m(cls: Type[T]) -> T: ...
class C(B):
@classmethod
def m(cls) -> C: ... |
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(As for a workaround you can use |
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@ilevkivskyi I see, thank you for the response. Just to clarify, the issue is generics in classmethods that return self-types? Is it the same issue here: #3645 ? I'm not sure silencing would be an option for me, because I would have to silence the error every time this generic container class is instantiated, because it never recognizes anything as fulfilling the protocol (unless I misunderstood your suggestion). I'll probably just refactor to work with an instance method. Thanks for the suggestion anyway. |
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This is different from #3645, but a proper fix will likely fix both. Yes, using an instance method is a good option. |
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Excuse me, but why the issue is closed? There is no solution how to use class methods in Protocols, i.e I have the following Protocol and implementation: from typing import Protocol, TypeVar, Dict, Type, Generic
from dataclasses import dataclass
ModelType = TypeVar("ModelType", bound="DictSerializable")
class DictSerializable(Protocol):
@classmethod
def from_dict(cls: Type[ModelType], data: Dict) -> ModelType:
...
def to_dict(self: ModelType) -> Dict:
...
@dataclass
class Item:
value: int
@classmethod
def from_dict(cls: Type[Item], data: Dict) -> Item:
return Item(value=data["value"])
def to_dict(self) -> Dict:
return {"value": self.value}
item: DictSerializable = Item(value=1)Error: Playground https://mypy-play.net/?mypy=latest&python=3.9&gist=cd5f30078d13e2cac70376a0bdc2ff06 The main problem that the DictSerializable structures possibly are not controlled by the |
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Hey @SergeyTsaplin,
I tried to reproduce the errors you mentioned in your comment with an upgraded version of your minimal example, incorporating the following changes:
The upgraded code: from typing import Protocol, TypeAlias, Type, TypeVar
from dataclasses import dataclass
ModelType = TypeVar("ModelType", bound="DictSerializable")
ItemDict: TypeAlias = dict[str, int]
class DictSerializable(Protocol):
@classmethod
def from_dict(cls: ModelType, data: dict) -> ModelType:
...
def to_dict(self: ModelType) -> dict:
...
@dataclass
class Item:
value: int
@classmethod
def from_dict(cls: Type['Item'], data: ItemDict) -> 'Item':
return Item(value=data["value"])
def to_dict(self) -> ItemDict:
return {"value": self.value}
item: DictSerializable = Item(value=1)
print(item)I can run this code without problems: $ mypy spam.py && python spam.py
Success: no issues found in 1 source file
Item(value=1)using |
A potential bug
So, I've posted a corresponding question on StackOverflow, but it hasn't gotten much traction.
Here is my basic situation: I'm trying to use a Protocol that would include a couple of classmethods.
The protocol is generic too, and the signatures involves self-types:
When I try to type-check this, I get the following errors:
Why are the return types registering as
<nothing>? When omit the annotation for theclsargument in the protocol, I get the following error:Which, quite frankly, makes even less sense to me.
If I make the classmethods instance methods, I receive no error. I would want to avoid this, since it would require a more hefty re-design.
If I try to use a staticmethod (which would be an easier workaround) it also fails:
error:
What is wrong with my Protocol / class definition? Am I missing something obvious?
I am using mypy version 0.620
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