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Why does math have an fabs function? Both it and the abs builtin function wind up calling fabs() for floats. abs() is faster to boot.

Nothing deep -- the math module supplies everything in C89's standard libm (+ a few extensions), fabs() is a std C89 libm function. There isn't a clear (to me) reason why one would be faster than the other; sounds accidental; math.fabs() could certainly be made faster (as currently implemented (via math_1), it endures a pile of general-purpose "try to guess whether libm should have set errno" boilerplate that's wasted (there are no domain or range errors possible for fabs())).