On Mon, Jul 29, 2013 at 10:20 AM, Chris Angelico <ros...@gmail.com> wrote: > On Mon, Jul 29, 2013 at 5:09 PM, MRAB <pyt...@mrabarnett.plus.com> wrote: >> I'm surprised that Fraction(1/3) != Fraction(1, 3); after all, floats >> are approximate anyway, and the float value 1/3 is more likely to be >> Fraction(1, 3) than Fraction(6004799503160661, 18014398509481984). > > At what point should it become Fraction(1, 3)?
At the point where the float is exactly equal to the value you get from the floating-point division 1/3. If it's some other float then the user didn't get there by entering 1/3, so it's not worth trying to pretend that they did. We do a similar rounding when formatting floats to strings, but in that case one only has to worry about divisors that are powers of 10. I imagine it's going to take more time to find the correct fraction when any pair of relatively prime integers can be a candidate numerator and denominator. Additionally, the string rounding only occurs when the float is being formatted for display; we certainly don't do it as the result of numeric operations where it could result in loss of precision. -- http://mail.python.org/mailman/listinfo/python-list
