mypy complains about classmethod

Question

I have a trivial dataclass (from pydantic)

from pydantic.dataclasses import dataclass

from abc import ABCMeta
from abc import abstractmethod


@dataclass
class BaseEntity(metaclass=ABCMeta):
    @classmethod
    @abstractmethod
    def from_dict(cls, other: dict):
        ...

    @abstractmethod
    def dict(self):
        ...


@dataclass
class UserEntity(BaseEntity):
    id: Optional[str]
    name: str
    email: str
    avatar: str

    @classmethod
    def from_dict(cls, other: dict):
        return cls(
            id=other.get("id"),
            name=other.get("name"),
            email=other.get("email"),
            avatar=other.get("avatar"),
        )

When I run mypy, I get this set of errors:

app/entities/user.py:25: error: Unexpected keyword argument "id" for "UserEntity" [call-arg]

app/entities/user.py:25: error: Unexpected keyword argument "name" for "UserEntity" [call-arg]

app/entities/user.py:25: error: Unexpected keyword argument "email" for "UserEntity" [call-arg]

app/entities/user.py:25: error: Unexpected keyword argument "avatar" for "UserEntity" [call-arg]

What I’m doing wrong? The code is fine; it runs. Or is it a mypy bug?

$ mypy --version
mypy 1.0.0 (compiled: yes)

Asked By: Rodrigo

||

Source

Answer 1

You’re missing the constructor from_dict is supposed to call.

def __init__(self, id, name, email, avatar):                                                                                      
        self.id = id                                                                                                                  
        self.name = name                                                                                                              
        self.email = email                                                                                                            
        self.avatar = avatar

Answered By: JustLearning

Answer 2

The same code with the standard library dataclasses module is fine.

The problem is that given the class definition alone, there is no indication that UserEntity.__init__ will accept any arguments, positional or keyword, because the only statically defined __init__ for mypy to check against is object.__init__.

However, mypy is coded to know about dataclasses from the standard library, and in particular what to expect from the decorator dataclasses.dataclass, so that it can recognize that there will be an __init__ method that has id, name, etc. as parameters.

This special treatment does not apply to pydantic.dataclasses, however.

Answered By: chepner

mypy complains about classmethod

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