pyrefly not liking Session as a context manager in sqlalchemy

[asukaminato0721]

Describe the Bug

ref

with Session(db_engine) as session

Cannot use Session as a context manager [bad-argument-type]

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[asukaminato0721]

err from ci

ERROR Object of class `TypeVar` has no attribute `begin` [missing-attribute]
   --> /home/runner/work/dify/dify/api/services/workflow_service.py:429:50
    |
429 |         with Session(bind=db.engine) as session, session.begin():
    |                                                  ^^^^^^^^^^^^^
    |
ERROR Argument `TypeVar[_S]` is not assignable to parameter `session` with type `Session` in function `services.workflow_draft_variable_service.DraftVariableSaver.__init__` [bad-argument-type]

[lolpack]

Hi @asukaminato0721 , thank you for raising the issue and awesome to see that you are giving Pyrefly a try. We are still creating a minimal reproduction of the issue, but there is indeed a problem with our TypeVar handling. Bug reports like these are very helpful - much appreciated :)

If interested, I included a way to suppress that for now below if it helps your testing. Below a sample config for the repo linked in this issue based on the mypy config:

project-excludes = [
   "tests/",
   "migrations/",
]
ignore-missing-imports = [
   "flask_login",
   "flask_restful",
   "flask_restful.inputs",
]

replace-imports-with-any = ["sqlalchemy.*"] # Until typevar bug is fixed https://github.com/facebook/pyrefly/issues/954 

untyped-def-behavior = "check-and-infer-return-any" # Corresponds to mypy behavior. You can enable it to get more powerful return inference, but more errors will come as a result

errors.implicitly-defined-attribute = false # Common pattern that while not totally safe, is widely used

You can put it at the same level as your pyproject.toml.

Also, if you are in the Pyrefly discord server, you can dm me here: pollackaaron_59375. I’d really like to learn more about what features matter to you for type checking.

[rchen152]

Here's a self-contained reproducer that doesn't use SQLAlchemy: https://pyrefly.org/sandbox/?code=GYJw9gtgBA%2BjwFcAuCQFM5QJYQA5hCSgEMA7UsJYpLMUgZwChHRIokBPXLUgc2zwEiAFS5oAasRDNGAYwA2xevSgBlNMtqkAXIyj6oAEzTBYMNKSRoQcABT0084NtiqAlFAC0APle6DAVDoKCCkUA5OzAHGpnBoAB5YSHYRwAA07GIwGQBuxPIIaBlIIMSyaABGZQDWbv6B%2BrhKTHoGMVAVaLw89o7Ada0NTcoyMKpQALxQorgSUrYARGMLGRVgCKSGEwvqmnQLbjIA7kkAFmoa9Fq2Hkrhl1r1%2Bg57pAB0nd2kN4xAA&version=3.12.

The problem seems to be that Pyrefly doesn't like TypeVar _S being defined after Session - if I move _S before Session, the errors disappear.