[Anders J. Munch] > What design error? float.__mod__ works perfectly. > > >>> -1 % 50 > 49 > >>> -1.0 % 50.0 > 49.0 > >>>
Please read the whole thread. Maybe you did, but you said nothing here that indicated you had. The issues aren't about tiny integers that happen to be in float format, where the result is exactly representable as a float too. Those don't create problems for any definition of mod. But even non-tiny exact integers can. > It's only Decimal.__mod__ that's inconsistent. float.__mod__ has the > usual floating-point inaccuracies, but then with float that goes with > the territory. No. Decimal.__mod_ always returns the mathematically exact result. It has this in common with math.fmod() (in fact, math.fmod() and Decimal.__mod__() have the same definition, ignoring IEEE endcases). It's impossible to do so under Python's integer-derived mod definition. Read the whole thread for why -- we can't even guarantee that abs(a%b) < abs(b) for non-zero finite floats under the current mod, and that has in fact been a source of complaints over the years. math.fmod and Decimal.__mod__ do guarantee abs(a%b) < abs(b) for all non-zero finite floats, and moreover guarantee that a%b is an exact integer multiple of `b` away from `a` (although that integer may not be representable as a float) -- again it's impossible for the a - floor(a/b)*b definition to do so for floats. > I've had occasion to use it, and it was a pleasant surprise that it > "just worked", so I didn't have to go to the math module and ponder > over the difference between modulo or remainder. There's no such distinction in Python's math module -- fmod is the only modular reduction function in the math module. The `decimal` module has two such functions (see earlier messages in this thread for examples -- neither matches Python's mod function). _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
