BTW, the StackOverflow question helped me understand the use-case here:

  http://stackoverflow.com/q/11154954/270986

The perspective is that of a *Fraction* user who wants to be able to easily see the Decimal expansion of a Fraction to an arbitrary number of decimal places.  (I was trying to understand why a *Decimal* user would care about converting from Fraction instances, but that's the wrong way around.)

That desire to have an easy way to see the digits of a Fraction seems reasonable to me, but I'm not sure that having a Decimal method is the right way to go about it.  Another possible solution would be to implement a decent __format__ method for Fraction, so that somebody could do:

>>> format(Fraction(1, 7), '.17g')
>>> '0.14285714285714286'