Unexpected results comparing float to Fraction

Chris Angelico rosuav at gmail.com
Mon Jul 29 18:20:27 CEST 2013


On Mon, Jul 29, 2013 at 5:09 PM, MRAB <python at 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)?

>>> Fraction(0.3)
Fraction(5404319552844595, 18014398509481984)
>>> Fraction(0.33)
Fraction(5944751508129055, 18014398509481984)
>>> Fraction(0.333)
Fraction(5998794703657501, 18014398509481984)
>>> Fraction(0.3333333)
Fraction(6004798902680711, 18014398509481984)
>>> Fraction(0.3333333333)
Fraction(6004799502560181, 18014398509481984)
>>> Fraction(0.3333333333333)
Fraction(6004799503160061, 18014398509481984)
>>> Fraction(0.33333333333333333)
Fraction(6004799503160661, 18014398509481984)

Rounding off like that is a job for a cool library function (one of
which was mentioned on this list a little while ago, I believe), but
not IMO for the Fraction constructor.

ChrisA



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