Doesn't find column on child class

[bochecha]

Here is a very simplified reproducer:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.schema import Column
from sqlalchemy.types import Integer


Base = declarative_base()


class BaseMixin(Base):
    __abstract__ = True

    field1 = Column(Integer)

    @classmethod
    def from_something(cls, something_complicated: int) -> 'BaseMixin':
        # Real code tries to figure out which model to use
        return MyModel(field1=something_complicated, field2=something_complicated)


class MyModel(BaseMixin):
    __tablename__ = 'first-model'

    field2 = Column(Integer)

With sqlalchemy-stubs from master, I get the following:

$ pipenv run mypy model.py 
model.py:17: error: Unexpected column "field2" for model "MyModel"

But field2 exists on the model. 😕

This seems to be due to the order of the code because if I add the following line at the end of the file, then Mypy doesn't report any error:

m = MyModel(field1=1, field2=2)

[bochecha]

The problem seems to be due to

sqlalchemy-stubs/sqlmypy.py

Line 214 in f6c1105

if isinstance(sym.node, Var) and isinstance(sym.node.type, Instance):

When Python passes over that line for BaseMixin.field1, both conditions are True.

But when it passes over it for MyModel.field2, the second condition is False because sym.node.type is None.

As a result, field2 doesn't get added to expected_types, which leads to the error.

I'm not sure how/where sym.node.type is supposed to be computed, maybe this is a bug in mypy instead?

[ilevkivskyi]

I think your diagnosis is correct, but this may be not easy to fix with the current state of mypy internals (we can't trigger a deferral from this point, because module top-levels can't be deferred).

Anyway, this is a known limitation in mypy, and it will likely be removed at some point soon.

[bochecha]

Anyway, this is a known limitation in mypy

Do you happen to have a link to a mypy issue handy?

[ilevkivskyi]

Here is the link python/mypy#6356