On: Many programming languages have infix expressions, but get associativity wrong

[jedi_stannis]

Yes it is really pulling a syntactic trick and not comparing against bools. 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.

http://docs.python.org/reference/expressions.html#notin