After exercising the fractions module, I've found it problematic that 
there isn't a unified constructor to handle mixed data input types.  

For example, when updating the accurate summation recipe at 
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/393090 to show 
how it could be done with rational arithmetic.  Handling mixed-type 
inputs required writing a factory function:

    def makefrac(x):
        if isinstance(x, Decimal):
            return Fraction.from_decimal(x)
        if isinstance(x, float):
            return Fraction.from_float(x)
        return Fraction(x)

That function supported a clean version of the recipe:

    def frsum(iterable):
        return float(sum(map(makefrac, iterable)))

It would have been much better if that functionality were built into 
the class definition.  See attached patch.