Idea: warn if an object without a "significant" boolean representation is used in a boolean context

[brianmaissy]

This is a bit of a long shot, because this isn't really a type error per se. But consider the following:

numbers: List[int] = []
my_generator: Iterable = (i for i in numbers)
if not my_generator:
    print("empty")

Obviously this will not print "empty", because a is a generator, which is always truthy. However, 99.9% of the times this is not what the user intended, and it would be great if a tool could identify it and warn about it, and mypy seems well-placed to be able to do it.

Beyond the fact that this isn't really a type "error", there are some more complications. For example, how to more generally define what objects don't have a "significant" boolean representation. One could argue that this is just as much a "mistake" as the example with the generator:

class A:
    pass

a: A = A()
if not a:
    print("empty")

As well as the question of what's considered "using something in a boolean context". Should calling bool() on a generator be the same as using it in the conditional expression of an if or while?

So I'm not at all sure this is a feasible or reasonable suggestion, but I didn't find any mention if it in past issues, so thought it was worth bringing up for discussion.

[JukkaL]

I've encountered similar issues myself, though I can't remember if there has been any online discussion about these. Stricter boolean checks would at least be nice as an opt-in check.

[hauntsaninja]

#10666 will add an opt-in check for this.

[Akuli]

Can this be closed, now that #10666 is merged?