MappedAsDataclass Optional generates non-Optional typing annotations

[BranislavBajuzik]

Ensure stubs packages are not installed

• No sqlalchemy stub packages is installed (both sqlalchemy-stubs and sqlalchemy2-stubs are not compatible with v2)

Verify if the api is typed

• The api is not in a module listed in #6810 so it should pass type checking

Describe the typing issue

When having MappedAsDataclass in the class hierarchy, Mypy reports Optional attributes as not Optional.

The generated DDL is correct, without the NOT NULL part

To Reproduce

from sqlalchemy.orm import MappedAsDataclass, DeclarativeBase, Mapped


class Test(MappedAsDataclass, DeclarativeBase):
    __tablename__ = "test"
    test_str_attribute: Mapped[str | None]


Test(test_str_attribute=None)

Error

error: Argument "test_str_attribute" to "Test" has incompatible type "None"; expected "Union[SQLCoreOperations[Optional[str]], str]"  [arg-type]

Versions

• OS: Kubuntu 22.04
• Python: 3.10.6
• SQLAlchemy: 2.0.15
• Type checker: mypy 1.3.0 (compiled: yes)

Additional context

This seems oddly similar to #9567, but the referenced mypy issue, python/mypy#13856, is marked as resolved in 1.2 (also in docs)

Thank you

[CaselIT]

Hi,

That's expected. If you try to run the above code it errors with:

sqlalchemy.exc.InvalidRequestError: Cannot use 'DeclarativeBase' directly as a declarative base class. Create a Base by creating a subclass of it.

Follow what the error suggests and it should work

[BranislavBajuzik]

I apologize for making the example unrunnable while trying to make a minimal example. I do not have much experience with SQLAlchemy, I am actually using it for the first time (I got excited about the typing 😅 )

I did subclass the DeclarativeBase and I still get this error:

As you can see, the database is created successfully, and the data is inserted, but Mypy is not happy. If I remove the MappedAsDataclass (and the init=False, pardon the screenshot), the error is gone:

Thank you

the code:

from sqlalchemy import create_engine
from sqlalchemy.orm import MappedAsDataclass, DeclarativeBase, Mapped, mapped_column, Session


class Base(DeclarativeBase):
    pass


class Test(MappedAsDataclass, Base):
    __tablename__ = "test"
    id: Mapped[int] = mapped_column(init=False, primary_key=True)
    test_str_attribute: Mapped[str | None]


engine = create_engine("sqlite:///db.sqlite", echo=True)

Base.metadata.create_all(engine)

with Session(engine) as s:
    t = Test(test_str_attribute=None)
    s.add(t)
    s.commit()

[CaselIT]

This is a bug of MyPy. pyright or pylance type check the code correctly. cc @zzzeek
Example:

from typing import Any, Generic, TypeVar, dataclass_transform, reveal_type

T = TypeVar("T")

class Desc(Generic[T]):
    def __set__(self, instance: Any, value: "Desc[T]| T") -> None:
        ...

@dataclass_transform(field_specifiers=(Desc,))
class DescMeta(type):
    pass

class DescBase(metaclass=DescMeta):
    pass

class Foo(DescBase):
    bar: Desc[int | None]

reveal_type(Foo.__init__)
Foo(bar=None)

This prints in mypy

test1.py:19: note: Revealed type is "def (self: test1.Foo, bar: Union[test1.Desc[Union[builtins.int, None]], builtins.int])"
test1.py:20: error: Argument "bar" to "Foo" has incompatible type "None"; expected "Union[Desc[Optional[int]], int]"
  [arg-type]
Found 1 error in 1 file (checked 1 source file)

while pyright correctly says

test1.py:19:13 - information: Type of "Foo.__init__" is "(self: Foo, bar: Desc[int | None] | int | None) -> None"

[CaselIT]

@zzzeek I don't remember if you already reported this one to mypy

[zzzeek]

yes it's python/mypy#15020

[zzzeek]

so if that's the issue, then dupe of #9617