Cannot assert a callable returns `None` when it must return `None` #17568

cjw296 · 2024-07-23T07:03:42Z

Bug Report

mypy prevents you making assertions about callables that must return None when they're called.
This is important as not everyone works in a type-checked context, and so library code must be robust to this.
In my case, I have a chain of callables, the ones in the middle take iterables and return iterables, but the one at the start must take no arguments and the one at the end must consume the upstream iterable and return nothing.

To Reproduce

def returns_none() -> None:
    pass

assert returns_none() is None

Expected Behavior

No mypy complaints

Actual Behavior

"returns_none" does not return a value (it only ever returns None) [func-returns-value]

Your Environment

Mypy version used: mypy 1.10.1 (compiled: yes)
Mypy command-line flags: none
Python version used: Python 3.12.1

The text was updated successfully, but these errors were encountered:

gandhis1 · 2024-07-23T14:16:43Z

The error is to prevent people from attempting to do things with the return values of functions that return None. Whether or not that thing is an assertion doesn't really seem relevant. As the point of Mypy is to statically verify things, supporting constructs used with dynamic type verification seem to be outside of the scope of Mypy. You can type: ignore[func-returns-value] if you need to.

At a minimum I'd argue this is a feature request, not a bug

cjw296 added the bug mypy got something wrong label Jul 23, 2024

hauntsaninja mentioned this issue Oct 24, 2024

Fix [func-returns-value] not being trigged when calling decorated functions #18015

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Cannot assert a callable returns `None` when it must return `None` #17568

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Cannot assert a callable returns None when it must return None #17568

Cannot assert a callable returns None when it must return None #17568

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Cannot assert a callable returns `None` when it must return `None` #17568

Cannot assert a callable returns `None` when it must return `None` #17568