How is string.find implemented in CPython?

The comment on the implementation has the following to say:

fast search/count implementation, based on a mix between boyer-moore and horspool, with a few more bells and whistles on the top.

for some more background, see: http://effbot.org/zone/stringlib.htm

—https://github.com/python/cpython/blob/master/Objects/stringlib/fastsearch.h#L5

You should be able to find it in Objects/stringlib/find.h, although the real code is in fastsearch.h.