See also:

    https://en.wikipedia.org/wiki/Hamming_weight

As that says, there are a number of languages and processors with first class support for a popcount function.  I've frequently implemented it in Python when using integers as integer bitsets (`i` is in the set if and only if bit `2**i` is set in the integer), which often - except for finding the cardinality - runs much faster than using general Python sets.