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inferred type of "(cond and a) or b" #1477
Description
If cond: Bool, a: A, and b: B, then mypy thinks (cond and a) or b has type Union[bool, A, B]. But in fact the type should just be Union[A, B]. The only way (cond and a) or b can be a bool is for it to be (cond and a), and for that to be truth-y; but in that case (cond and a) would have type A, and never type bool (from cond).
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rwbarton commentedon Jun 13, 2016
See also discussion at #1698.
With
FalseandTruetypes we could type check this as follows: withcond: bool,a: A,b: B, we first havecond and a: Union[falsey_type(bool), A] = Union[False, A], and then(cond and a) or b: Union[truthy_type(Union[False, A]), B] = Union[A, B].dmoisset commentedon Aug 5, 2016
Just checked that my PR #1989 happens to fix this too
ddfisher commentedon Aug 18, 2016
Fixed by #1989.