[truthy-function] does not work with @cache

[socketpair]

from functools import cache

def fun1():
    pass

if fun1:  # bug here is caught, nice!
    print(42)

@cache
def fun2():
    pass

if fun2:  # Fails to find a bug!
    print(42)

All messages:

main.py:6: error: Function "fun1" could always be true in boolean context  [truthy-function]
Found 1 error in 1 file (checked 1 source file)

[JelleZijlstra]

This is closely related to python/typeshed#11280 but could perhaps be fixed in mypy independently. If you add a reveal_type(fun2) it says Revealed type is "functools._lru_cache_wrapper[Any]"; mypy could be enhanced to warn on boolean use of that type.

[socketpair]

@JelleZijlstra possibly consider any callable? Yes, incorrect in general, but correct in practice.

Or, better, maybe check anything that does not have explicit __bool__, not None, not a string, number or standard container? Possibly with some exceptions for common cases like regexp match (does it have __bool__ ?)

I think, any case of usage of an object in a condition without defined conversion to bool is a bug.

[JelleZijlstra]

Yes, a generalization could make sense. I implemented something like that in pyanalyze and it's been useful, but it's not a simple change to make.

[hauntsaninja]

I think mypy already supports that, just is disabled by default: https://mypy.readthedocs.io/en/stable/error_code_list2.html#check-that-expression-is-not-implicitly-true-in-boolean-context-truthy-bool

[socketpair]

Seems no such option here...

[hauntsaninja]

Closing per my previous comment. If you want to test it in mypy-play.net, you can use an inline comment at the top of the file like # mypy: enable-error-code="truthy-bool"