Hmm, you can reuse this in the RationalFraction constructor, isn't? First three if/elif's share exactly same logic.

Do you mean the main Fraction constructor?

There is a different exception if the argument type is not supported. There is also a matter of performance.

I meant some helper:

@classmethod
def _from_number(cls, number):
    if type(number) is int:
        return cls._from_coprime_ints(number, 1)
    elif isinstance(number, numbers.Rational):
        return cls._from_coprime_ints(number.numerator, number.denominator)
    elif (isinstance(number, float) or
          (not isinstance(number, type) and
           hasattr(number, 'as_integer_ratio'))):
        return cls._from_coprime_ints(*number.as_integer_ratio())

Then you can reuse one in the constructor and in the from_number(), e.g.:

        if denominator is None:
            self = cls._from_number(numerator)
            if self is not None:
                return self

            elif isinstance(numerator, str):            
                ...

There is also a matter of performance.

I don't expect too much from extra class method call.

Extra function call and extra check.

This will not save much code. I'll leave this on future if we touch this code again.

Well, it's up to you. PR has Mark approval, so it's ready to go.