The old sqlalchemy-stubs from dropbox included an approximation for the behavior of mapped classes using declarative setup, that is

class Foo(Model):
  a = Column(DateTime)

Foo.a
# type is Column[DateTime]
Foo().a
# type is datetime

They did so by defining the get method on Column here: https://github.com/dropbox/sqlalchemy-stubs/blob/master/sqlalchemy-stubs/sql/schema.pyi#L131

Maybe the same can be done for set, so that

Foo().a = 3
# wrong type, expected datetime object

Of course the whole thing is supported already when using the mypy plugin, but for example when using pylance (vscode type engine) which does not support plugins, it gets confused when using and assigning object values as mapped columns and complains about mismatching types.

I think the same is done on relationship() property

I'm unsure of the deeper implications of adding such type hints, though