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I guess it's included as an easy way to access the fast hardware supported implementation, on architectures where it is available.

It does. Note that it calls popcount_digit on each digit of the integer (a 'digit' here is 32 bits), which calls _Py_popcount32, which does map to a hardware instruction if possible: https://github.com/python/cpython/blob/f62ad4f2c4214fdc05cc4...

Ah, you are correct. Although I still take issue with "fast" in the comment I was replying to ;)