On Tue, Jun 12, 2018 at 4:40 PM, Greg Ewing <greg.ew...@canterbury.ac.nz> wrote: > Tim Peters wrote: > >> 1. Python's float "%" is unsuitable for argument reduction; e.g., >> >> >>> -1e-14 % 360.0 >> 360.0 >> >> `math.fmod` is suitable, because it's exact: >> >> >>> math.fmod(-1e-14, 360.0) >> -1e-14 > > > So why doesn't float % use math.fmod?
https://docs.python.org/3/reference/expressions.html#binary-arithmetic-operations https://docs.python.org/3/reference/expressions.html#id17 https://docs.python.org/3/reference/expressions.html#id18 (the latter two being footnotes from the section in the first link) With real numbers, divmod (and thus the // and % operators) would always return values such that: div, mod = divmod(x, y): 1) div*y + mod == x 2) sign(mod) == sign(y) 3) 0 <= abs(mod) < abs(y) But with floats, you can't guarantee all three of these. The divmod function focuses on the first, guaranteeing the fundamental arithmetic equality, but to do so, it sometimes has to bend the third one and return mod==y. There are times when it's better to sacrifice one than the other, and there are other times when it's the other way around. We get the two options. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
