Why does the SQLAlchemy code distinguish between bound=object and bound=Any?

[pamelafox]

I'm working on a Flask-SQLAlchemy PR to improve the typing, and am basing some types on how SQLAlchemy does it. I'm puzzled over this:

# I would have preferred this were bound=object however it seems
# to not travel in all situations when defined in that way.
_O = TypeVar("_O", bound=Any)
"""The 'ORM mapped object' type.
"""

_OO = TypeVar("_OO", bound=object)
"""The 'ORM mapped object, that's definitely object' type.
"""

From
https://github.com/sqlalchemy/sqlalchemy/blob/2dced78f33010d4a6a6276f4e8c5665bd08d01c0/lib/sqlalchemy/orm/_typing.py

In what situations does _OO not work? It seems that _O is used in most of the signatures.

Thanks!

[zzzeek]

I recall coming across this difference but i dont recall what the error was, it was also an older version of mypy. I will say that typing tools are very liberal when they see Any in a different way when they see any other type, even object which is the base type of literally everything in the Python interpreter.

Let's try right now to see if theres any difference; first apply this diff:

diff --git a/lib/sqlalchemy/orm/_typing.py b/lib/sqlalchemy/orm/_typing.py
index 36dc6ddb9c..c433d98a72 100644
--- a/lib/sqlalchemy/orm/_typing.py
+++ b/lib/sqlalchemy/orm/_typing.py
@@ -55,7 +55,7 @@ _T_co = TypeVar("_T_co", bound=Any, covariant=True)
 
 # I would have preferred this were bound=object however it seems
 # to not travel in all situations when defined in that way.
-_O = TypeVar("_O", bound=Any)
+_O = TypeVar("_O", bound=object)
 """The 'ORM mapped object' type.
 
 """

then let's run the pep-484 suite - here's the errors we get:

$ tox -e pep484
# ...
lib/sqlalchemy/orm/state.py:604: error: Incompatible types in assignment (expression has type "List[Any]", target has type "Union[Set[str], Dict[str, Any], Type[_O], _O, None]")  [assignment]
lib/sqlalchemy/orm/state.py:606: error: Incompatible types in assignment (expression has type "_SerializeManager", target has type "Union[Set[str], Dict[str, Any], Type[_O], _O, None]")  [assignment]
lib/sqlalchemy/orm/util.py:837: error: Item "object" of "Union[_O, AliasedClass[_O]]" has no attribute "__name__"  [union-attr]
lib/sqlalchemy/orm/util.py:1296: error: Item "object" of "Union[_O, AliasedClass[_O]]" has no attribute "__name__"  [union-attr]
lib/sqlalchemy/orm/util.py:1304: error: Item "object" of "Union[_O, AliasedClass[_O]]" has no attribute "__name__"  [union-attr]
lib/sqlalchemy/orm/decl_api.py:1607: error: "Type[_O]" has no attribute "_sa_apply_dc_transforms"  [attr-defined]
lib/sqlalchemy/ext/asyncio/session.py:593: error: Incompatible return value type (got "Optional[_O]", expected "Optional[_O]")  [return-value]

reverting that patch and running pep-484 again the errors go away.

[pamelafox]

Thanks @zzzeek! I'm new to typing in Python, so I also asked generally about bound= in the Python forum, if you're curious to see that discussion:
https://discuss.python.org/t/differences-in-bound-object-vs-bound-any/27052

For the three errors about "name", I wonder if a TypeVar with bound=type would be appropriate there? I think those are cases where the variable is a class that's an instance of type, and even the SQLAlchemy metaclasses like _DynamicAttributesType inherit from type. This passes mypy:

from typing import Any, TypeVar

A = TypeVar('A', bound=Any)
O = TypeVar('O', bound=type)


def thing1(a: A):
    a.__name__


def thing2(a: O):
    a.__name__

This is just musing based on what little I know of typing, and me trying to understand it better. It's also not a big deal, so I appreciate your response. :)

[zzzeek]

I think you're correct, the ones that refer to __name__ are actually referring to "instance of types", the way AliasedClass is mostly like a Type object; here that would be written as:

Union[Type[_O], AliasedClass[_O]]

that is, _O could be object as we want it to be.

anyway yeah we could likely fix this _O thing since that's very few errors

[zzzeek]

see https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/4637 where I have all of these errors resolved. thanks for the tip!

[CaselIT]

Last one is definitely the best error :/