How can I check Hamming Weight without converting to binary?

You cannot make this computationally less complex. It will be O(n) the number of bits, or, as the answer with the & trick showed, O(n) the number of bits set to 1; but unless all of the numbers you are using are a special case, the latter should on average be n/2, so both of those O(n) numbers are the same.

And the lookup-table trick, of course, is actually doing nothing for the computational complexity; it's just paying for time with space but without changing the underlying economics, which are that you must examine each bit once somehow and there is no way around that. You cannot, logically, answer a question about the bits in the number without inspecting each of them.

Now, I suppose I'm being a bit sloppy since many of these examples are actually O(n^2) since in Python you have to examine the whole number at once time, so with a Python long integer of, say, 100 bytes, a + or an & or a / operation will look at each byte at least once, and that will happen over and over until the number is reduced to zero (in the schemes outlined above), so these, again, are really O(n^2) operations. I am not sure Python will allow a true O(n) solution here.

Anyway: if you were really asking about computational complexity, which specifically means big-O analysis, that's your answer. :-)

I'm not a python programmer, but hopefully it will be enough for you to follow.

c = 0
while n:
    c += 1
    n &= n - 1

return c

While a little obscure, it's primary advantage is speed and simplicity. The while loop is only iterated once for every bit set to 1 in n.