False warning: Returning Any from function declared to return "bool"

[stefanhoelzl]

• Are you reporting a bug, or opening a feature request? Bugreport
• Please insert below the code you are checking with mypy

from typing import Any

class Structure():
    def __eq__(self, other: Any) -> bool:
        return {} == other

• What is the actual behavior/output?

mypytest.py:5: warning: Returning Any from function declared to return "bool"

• What is the behavior/output you expect? No error
• What are the versions of mypy and Python you are using?

Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 05:52:31) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin

mypy==0.630

• What are the mypy flags you are using?
  --strict

[ilevkivskyi]

Why do you think the warning is false? It looks like a totally reasonable warning, since --warn-return-any is part of --strict. If you don't like it, don't use this flag.

[stefanhoelzl]

because {} == other should evaluate to a bool.
Which means the method returns a bool.
Then getting a warning Returning Any seems false to me.

[ilevkivskyi]

Everything applied to Any evaluates to Any. A dynamic class can define __eq__ that returns a string, there is no guarantee it is a bool.

[ylathouris]

Everything applied to Any evaluates to Any. A dynamic class can define __eq__ that returns a string, there is no guarantee it is a bool.

This was helpful. Thank you!

[Torxed]

I get the same error when using:

def verify_password(plain_password :str, hashed_password :str) -> bool:
	return pwd_context.verify(plain_password, hashed_password)

Which is taken from the FastAPI docs. In this case .verify() is a verification function that to the best of my knowledge can only return True or False but is not typed: https://foss.heptapod.net/python-libs/passlib/-/blob/01dfe753e4f4590307611ef5c6304279a7364305/passlib/context.py?page=3#L2272-2313

I think I understand the rules of the Any evaluation, but perhaps I've misunderstood how mypy evaluates these cases.
Shouldn't the return value matter in mypy's checks? That is that it's a static value of False or True that gets returned?

In any case, the fix is simple enough:

def verify_password(plain_password :str, hashed_password :str) -> bool:
	return bool(pwd_context.verify(plain_password, hashed_password))

But it feels excessive to enforce one additional function call per call stack to please mypy, even in --strict mode.
It's the first time trying to use linters and type checkers, and this thread came up. I assumed it would deal with these cases "smartly", is that not the case?

[JelleZijlstra]

@Torxed the fix seems simple here: mypy needs to be told what the return type of .verify() is. If there's no annotation, it doesn't know.

I would not recommend turning on the option that generates this error. It's too strict to be useful for most code.

[hauntsaninja]

--no-warn-return-any :-)