Unexpected results comparing float to Fraction

[Serhiy Storchaka]

29.07.13 19:09, MRAB написав(ла):
> 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).

 >>> def approximate_fraction(f):
     prev_numer, numer = 0, 1
     prev_denom, denom = 1, 0
     r = f
     while True:
         i = math.floor(r)
         prev_numer, numer = numer, i * numer + prev_numer
         prev_denom, denom = denom, i * denom + prev_denom
         if i == r or numer / denom == f:
             break
         r = 1 / (r - i)

     return Fraction(numer, denom)

 >>> approximate_fraction(1/3)
Fraction(1, 3)
 >>> approximate_fraction(1e-17)
Fraction(1, 100000000000000000)
 >>> approximate_fraction(math.pi)
Fraction(245850922, 78256779)

I guess the Fraction constructor is more faster than this function.