Why does sqlalchemy not infer the correct type if the custom type is used with a module import?

[danielmoessner]

I've got the following init.py file in src/database:

from .uuid import UUID

And I've got the following custom type in src/database/uuid.py:

if TYPE_CHECKING:
    UUIDTypeDecorator = TypeDecorator[uuid.UUID]
else:
    UUIDTypeDecorator = TypeDecorator

class UUID(UUIDTypeDecorator):
    """
    Platform-independent UUID type.
    Uses PostgreSQL's UUID type, otherwise uses CHAR(32), storing as stringified hex values.
    Taken from SQLAlchemy docs: https://docs.sqlalchemy.org/en/13/core/custom_types.html#backend-agnostic-guid-type
    """

    impl = CHAR
    cache_ok = True

    ...

Now I get an error when I try to use it as database.UUID but not with UUID:

Code:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column
from uuid import UUID

from src.database.uuid import UUID as DUUID
from src import database

Base = declarative_base()

class BaseModel(Base):
    __abstract__ = True

    id: UUID = Column(database.UUID, primary_key=True, index=True)
    user_id: UUID = Column(DUUID, index=True, nullable=False)

Why does mypy work correctly with user_id: UUID = Column(DUUID, index=True, nullable=False) but not with id: UUID = Column(database.UUID, primary_key=True, index=True)?

[danielmoessner]

A circular import error seems to have caused the problem. Interesting how this changed the type inference.