Possible `truthy-bool` false positive with `:=`

[sobolevn]

Consider this code with truthy-bool:

class A:
    def method(self) -> bool:
        return True  # whatever


if (a := A()) and a.method():
    print(a)

It will output:

» mypy ex.py --enable-error-code=truthy-bool
ex.py:6: error: "ex.a" has type "A" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
ex.py:6: error: "ex.A" returns "A" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]

Two questions:

1. Why two errors?
2. Maybe we should skip this rule completely for (:=) and ..., because a is not really the main thing we check here?

CC @ikonst

[AlexWaygood]

I think it would probably be better to emit one error rather than two here. But I don't think I agree that it's a false positive. The first side of the left operand will always evaluate to True, so you could write the snippet more simply as:

class A:
    def method(self) -> bool:
        return True  # whatever


if (a := A()).method():
    print(a)

[ikonst]

Agree with (1), need to look into it.

Don't see compelling evidence to special-case (2). This code uses and creatively as a statement separator (since lhs is always true), and I'm not convinced this is idiomatic or common (just browsed over examples in PEP-572 but I might've missed something).