Compatibility of "ordinary" and Optional types

[hubernhard]

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

Hi,

in this simple example I try to assign values from a Mapped[int] column into a Mapped[int | None] column. MyPy complains that those types are not compatible. I would assume it should behave similar to ordinary dataclasses where this is possible?

Thank you!

To Reproduce

from sqlalchemy.orm import DeclarativeBase, Mapped, MappedAsDataclass


class Base(MappedAsDataclass, DeclarativeBase):
    pass


class A(Base):
    x: Mapped[int | None]


class B(Base):
    x: Mapped[int]

    def export(self) -> A:
        return A(x=self.x)

Error

error: Argument "x" to "A" has incompatible type "Mapped[int]"; expected "Mapped[Optional[int]]"  [arg-type]

Versions

• OS: Windows
• Python: 3.10
• SQLAlchemy: 2.0.7
• Type checker: mypy 1.1.1

Additional context

No response

[zzzeek]

hi this is unfortunately a bug in mypy , this issue is a dupe of #9467.

mypy issue is at: python/mypy#13856

SQLAlchemy workarounds documented at : https://docs.sqlalchemy.org/en/20/orm/dataclasses.html#declarative-dataclass-mapping