bit count or bit set && Python3

[MRAB]

On 2012-10-25 15:47, Charles Hixson wrote:
> In Python3 is there any good way to count the number of on bits in an
> integer (after an & operation)?
> Alternatively, is there any VERY light-weight implementation of a bit
> set?  I'd prefer to use integers, as I'm probably going to need
> thousands of these, if the tests work out.  But before I can test, I
> need a decent bit counter.  (shift, xor, &, and | are already present
> for integer values, but I also need to count the number of "true" items
> after the logical operation.  So if a bitset is the correct approach,
> I'll need it to implement those operations, or their equivalents in
> terms of union and intersection.)
>
> Or do I need to drop into C for this?
>
There's a nice algorithm for counting the ones. It takes one iteration
per set bit:

def count_set_bits(number):
     count = 0
     while number:
         count += 1
         number &= number - 1
     return count