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

[chespinoza]

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]

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

[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:
python/mypy#7901

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

[tomdottom]

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

mypy is able to recognize the following

from typing import NewType

Email = NewType("Email", str)

class MySchema:
    email: Email

Instead of returning a function maybe marshmallow_dataclass.NewType could do one of the following:

A) Return a typing.NewType and internally map this to construction functions which can be looked as needed.

B) Construct and return a new Field Class.

[lovasoa]

A) Return a typing.NewType and internally map this to construction functions which can be looked as needed.

This is not possible. The logic for typing.NewType is hardcoded in mypy and does not support dynamic arguments. See the mypy issue I linked to above.

B) Construct and return a new Field Class.

This wouldn't work with the rest of marshmallow-dataclass. In particular, this wouldn't allow:

class MySchema:
    email: Email

[lovasoa]

@tomdottom : Would you be interested in writing a mypy plugin ?

[selimb]

I'd like for this to get fixed as well.
Wrote mypy plugin in #55