python / python/mypy

`return await ...` not working properly

Open
#18,805 1 comment 2 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug topic-type-context
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

Bug Report

Mypy incorrectly raises an error when i return await ...

To Reproduce

I cant seem to reproduce without using SQLAlchemy, but here is an MRE with it. Its possible this issue belongs in SQLAlchemy, however i was more inclined to ask here because the second implementation works correctly

from __future__ import annotations

from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column


class Base(DeclarativeBase):
    pass


class Model(Base):
    __tablename__ = "model"

    id: Mapped[int] = mapped_column(autoincrement=True, primary_key=True)


engine = create_async_engine("sqlite+aiosqlite:///:memory:")
session = AsyncSession(engine)


async def get_by_id(id: int) -> Model | None:
    return await session.scalar(select(Model).where(Model.id == id))

I get the following error for get_by_id:
error: Returning Any from function declared to return "Model | None" [no-any-return]

If i change the implementation to this though:

async def get_by_id(id: int) -> Model | None:   
    result = await session.scalar(select(Model).where(Model.id == id))
    return result

The error dissapears

Expected Behavior

Mypy should not show an error for the first implementation

Actual Behavior

It does

Your Environment

  • Mypy version used: 1.15.0
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files):
[tool.mypy]
python_version = "3.13"
disallow_untyped_defs = true
disallow_any_unimported = true
no_implicit_optional = true
check_untyped_defs = true
warn_return_any = true
warn_unused_ignores = true
show_error_codes = true
  • Python version used: 3.13.2

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the supplied Python reproducer using SQLAlchemy's AsyncSession.scalar and compare the inline await with the local-variable form under mypy 1.15.0 and the shown warn_return_any configuration. Trace how mypy handles the awaited expression and verify the fix with a regression test showing that both forms avoid the no-any-return error.

Written by the indexing model from the issue text.

Assessment

Tech stack
python, sqlalchemy, sqlite
Domain
databases, devtools, tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.