Mypy error: Variable "Email" is not valid as a type mypy(error)

Taking the example from the readme:

import marshmallow
from marshmallow_dataclass import NewType

Email = NewType("Email", str, field=marshmallow.fields.Email)

When using Email in a schema, for instance:

@marshmallow_dataclass.dataclass
class MySchema:
    id: str = field(metadata={"validate": marshmallow.validate.uuid})
    email: Email
    Schema: ClassVar[Type[marshmallow.Schema]] = marshmallow.Schema

I’m getting this error from mypy:

Mypy error: Variable "Email" is not valid as a type mypy(error)

versions:

marshmallow==3.2.1
marshmallow-dataclass==7.0.0

Thanks in advance.

[lovasoa]

I opened an issue at mypy, and they do not seem to be keen on implementing anything that would allow us make NewType work with mypy: https://github.com/python/mypy/issues/7901

I am going to leave this issue open, and if anyone is interested in writing a mypy plugin, we’ll track progress here.

[lovasoa]

Correct me if I’m wrong, but don’t think this is related to marshmallow_dataclass. mypy cannot handle any dynamically created types.

Your code can be reduced to the following, that mypy also rejects:

from typing import Type

Email: Type[str] = str

class MySchema:
    email: Email

This also gives: error: Variable "Email" is not valid as a type