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 equival

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