Appeasing MyPy trying to use an instance variable

Question

If I have the file mypytest/models.py that looks like this:

from typing import Type


class FakeBaseModel:
    pass


class FakeDatabase:
    Base: Type[FakeBaseModel]

    def __init__(self) -> None:
        self.Base = FakeBaseModel


db = FakeDatabase()


class FakeModel(db.Base):
    pass

And I run mypy ., I get the following output:

[mypytest {mypytest}]$ mypy .
mypytest/models.py:18: error: Name "db.Base" is not defined
Found 1 error in 1 file (checked 2 source files)

Clearly, though, db.Base is defined. How do I get mypy to recognize it?

Answer 1

I believe the problem here is that db.Base is a variable, not a type alias.

It's unclear from your example exactly why you need a dynamic base class. Inlining FakeBaseModel would obviously work; you could also just use a type alias while running the type checking:

from typing import Type, TYPE_CHECKING

# *snip*

db = FakeDatabase()

if TYPE_CHECKING:
    FakeBase = FakeBaseModel
else:
    FakeBase = db.Base

class FakeModel(FakeBase):
    pass

This will produce valid type checking based on what you've provided (basically, as long as FakeModel only uses what's in FakeBaseModel so it's safe to treat it as one).