In Python 'not in' membership operator evaluates to true if it does not finds a variable in the specified sequence and false otherwise. For example
>>> a = 10 >>> b = 4 >>> l1 = [1,2,3,4,5] >>> a not in l1 True >>> b not in l1 False
Since 'a' doesn't belong to l1, a not in b returns True. However, b can be found in l1, hence b not in l1 returns False