Testing equality of three values

Python chains such relational operators naturally (including in and is).

a() == b() == c() is functionally equivalent to a() == b() and b() == c() whenever consecutive calls to b return the same value and have the same aggregate side effects as a single call to b. For instance, there is no difference between the two expressions whenever b is a pure function with no side-effects.

The easiest way to show the slight difference:

>>> print(1) == print(2) == print(3)
1
2
3
True
>>> print(1) == print(2) and print(2) == print(3)
1
2
2
3
True

print() always returns None, so all we are doing is comparing Nones here, so the result is always True, but note that in the second case, print(2) is called twice, so we get two 2s in the output, while in the first case, the result is used for both comparisons, so it is only executed once.

Yes, however, when the comparisons are chained the common expression is evaluated once, when using and it's evaluated twice. In both cases the second comparison is not evaluated if the first one is false, example from the docs:

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).