Question

I’ve recently stumbled over this expression:

True == False in (False,)

It evaluates to False, but I don’t understand why.
True == False is False and False in (False,) is True, so both (to me) plausible possibilities

True == (False in (False,))

and

(True == False) in (False,)

evaluate to True, as I would have expected.
What is going wrong here?

Answer 1

I believe this is a corner case of Python’s comparison-operator chaining. It gets expanded to

 (True == False) and (False in (False,))

which evaluates to False.

This behavior was intended to match conventional math notation (e.g. x == y == z meaning that all three are equal, or 0 <= x < 10 meaning x is in the range [0, 10)). But in is also a comparison operator, giving the unexpected behavior.