gh-121797: Add class method Fraction.from_number()

[serhiy-storchaka]

It is an alternative constructor which only accepts a single numeric argument. Unlike to Fraction.from_float() and Fraction.from_decimal() it accepts any real numbers supported by the standard constructor (int, float, Decimal, Rational numbers).
Unlike to the standard constructor, it does not accept strings.

• Issue: Add class method Fraction.from_number() #121797

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[mdickinson]

+1 for the general idea. I haven't reviewed line-by-line.

[skirpichev]

LGTM with one nitpick (feel free to ignore)

[skirpichev]

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

[serhiy-storchaka]

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.

[skirpichev]

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.

[serhiy-storchaka]

Extra function call and extra check.

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

[skirpichev]

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