Asynchronous sessionmaker initialization type error

[r4rdsn]

Describe the bug
mypy fails on asynchronous sessionmaker initialization.

Expected behavior
No errors.

To Reproduce
Please try to provide a Minimal, Complete, and Verifiable example.
See also Reporting Bugs on the website, and some example issues.

Please do not use Flask-SQLAlchemy or any other third-party extensions or dependencies in test cases. The test case must illustrate the problem without using any third party SQLAlchemy extensions. Otherwise, please report the bug to those projects first.

import os

from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.orm import sessionmaker

engine = create_async_engine(os.getenv("DATABASE_URL"))
async_session = sessionmaker(engine, class_=AsyncSession)

Error

session.py:8: error: Value of type variable "_TSessionMakerType" of "sessionmaker" cannot be "AsyncSession"
Found 1 error in 1 file (checked 1 source file)

Versions.

• OS: Linux
• Python: 3.9.6
• SQLAlchemy: 1.4.23
• mypy: 0.910
• SQLAlchemy2-stubs: v0.0.2a9

Additional context
The problem appeared on the latest release of sqlalchemy2-stubs and was not present on v0.0.2a8.

Have a nice day!

[CaselIT]

Thanks for the report

[joaosr]

Same happening for me.

[CaselIT]

released as v0.0.2a10

[mjpieters]

This appears to have regressed? I'm using version 0.0.2a27, and the example in this issue now produces:

Argument of type "Type[AsyncSession]" cannot be assigned to parameter "class_" of type "Type[_TSessionMakerType@sessionmaker]" in function "__init__"
  Type "Type[AsyncSession]" cannot be assigned to type "Type[_TSessionMakerType@sessionmaker]"

when using Pyright.

I noticed that almost the same error message appears when using sqlalchemy.create_engine() and sqlalchemy.orm.Session:

Argument of type "Type[Session]" cannot be assigned to parameter "class_" of type "Type[_TSessionMakerType@sessionmaker]" in function "__init__"
  Type "Type[Session]" cannot be assigned to type "Type[_TSessionMakerType@sessionmaker]"

[zzzeek]

@mjpieters I am hoping 2.0 betas are very soon. that's where these issues if you have them will be permanently fixed so i hope you have time to test a bit.

[mjpieters]

Yeah, I can't find a release of sqlalchemy2-stubs that actually makes the example work, so it's probably a pyright incompatibility somewhere.

[CaselIT]

If you want to send a PR to fix things until v2 is out we can merge and release fairly quickly

[mjpieters]

If you want to send a PR to fix things until v2 is out we can merge and release fairly quickly

I traced this down to pyright not accepting that AsyncSession implements the TypingAsyncSessionProtocol type, because the signature of no_autoflush doesn't match:

Expression of type "Type[AsyncSession]" cannot be assigned to declared type "TypingAsyncSessionProtocol"
  "AsyncSession" is incompatible with protocol "TypingAsyncSessionProtocol"
  Type "Type[AsyncSession]" cannot be assigned to type "TypingAsyncSessionProtocol"
    "no_autoflush" is an incompatible type
      Type "(self: _TSessionNoIoTypingCommon@no_autoflush) -> ContextManager" cannot be assigned to type "(self: _TSharedSessionProtocol@no_autoflush) -> ContextManager"
        Parameter 1: type "_TSharedSessionProtocol@no_autoflush" cannot be assigned to type "_TSessionNoIoTypingCommon@no_autoflush"
          Type "_TSharedSessionProtocol@no_autoflush" cannot be assigned to type "_SessionNoIoTypingCommon[Unknown]"
        Function return type "ContextManager" is incompatible with type "ContextManager"
          "contextlib.AbstractContextManager" is incompatible with "contextlib.AbstractContextManager"

The issue could trivially be fixed if PEP 673 typing.Self / typing_extensions.Self was used there, but mypy doesn't yet support PEP 673. :-( I have not yet figured out what exactly is different between the context manager declarations or why using def no_autoflush(self) -> ContextManager[Self]: ... fixes the issue.

However, I think I can at least make this fail for mypy as well with:

from typing import TYPE_CHECKING, Type

from sqlalchemy.ext.asyncio import async_scoped_session
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.orm import sessionmaker

if TYPE_CHECKING:
    from sqlalchemy.ext.asyncio.session import TypingAsyncSessionProtocol

engine = create_async_engine(...)
SessionType: Type[TypingAsyncSessionProtocol] = AsyncSession
SM = sessionmaker(engine, class_=SessionType)

async_session = AsyncSession(engine)

as_session = async_scoped_session(SM, current_task)

This is an update to a single test/files/async_stuff.py line, pulling out the class_ argument into a separate type variable. Pyright / Pylance will complain about the SessionType assignment and again about passing SM on to async_scoped_session(), while MyPy only complains about the latter:

.../test/files/async_stuff.py:19: error: Argument 1 to "async_scope..._" has type "Callable[[KwArg(Any)], _AsyncSessionProtocol]"

[mjpieters]

Right, so the simplest fix is to not use bound typevars for methods producing a contextmanager; for both _SharedSessionProtocol and _SessionNoIoTypingCommon use:

    def no_autoflush(self: _M) -> ContextManager[_M]: ...

It's essentially the same meaning; whatever no_autoflush() returns, is a ContextManager that'll produce self when you call __enter__ on it, and it won't affect the implementation as the stub is separate.

I was, however, wrong about my assessment that I could make mypy fail on the same issue; the error it produces when you use the Type[TypingAsyncSessionProtocol] annotation sticks even when Pylance is now happy with the annotations.

[mjpieters]

I've traced this to a Pyright issue with @property and ContextManager: microsoft/pyright#4034

[CaselIT]

Thanks for debugging this. If you could provide a PR with the fix it would be great!

[mjpieters]

Thanks for debugging this. If you could provide a PR with the fix it would be great!

It's an acknowledged pyright bug; it's in their court. The type definition is correct, the type checker is not, so no PR needed!

[CaselIT]

oh ok, good to know! thanks for looking into it! We can close then!