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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.