[Python-Dev] Symmetry arguments for API expansion (original) (raw)

Tim Peters tim.peters at gmail.com
Tue Mar 13 12:37:00 EDT 2018

[Tim]

At heart, the Fraction() constructor is all about creating integer ratios, so is the most natural place to put knowledge of how to do so. A protocol for allowing new numeric types to get converted to Fraction would be more generally useful than just a weird method only datetime uses ;-)

[Guido]

Ironically, the various Fraction constructors calls asintegerratio() for floats and Decimals. From which follows IMO that the float and Decimal classes are the right place to encapsulate the knowledge on how to do it.

It appears that as_integer_ratio was slammed into floats and Decimals precisely so that Fraction() could call them, while Fraction has its own self-contained knowledge of how to convert ints and Fractions and strings and numbers.Rationals to Fraction (and the former types don't support as_integer_ratio).

That's fine, but my objection is subtler: the actual answer to "can this thing be converted to an integer ratio?" is not "does it support as_integer_ratio?", but rather "can Fraction() deal with it?" - and there's currently no way for a new numeric type to say "and here's how I can be converted to Fraction".

An obvious way to extend it is for Fraction() to look for a special method too, say "_as_integer_ratio()". The leading underscore would reflect the truth: that this wasn't really intended to be a public method on its own, but is an internal protocol for use by the Fraction() constructor.

Then it would be obvious that, e.g., it would be just plain stupid ;-) for int to bother implementing _as_integer_ratio. The only real point of the method is to play nice with the Fraction constructor. As is, it's jarring that int.as_integer_ratio() doesn't exist - for the same reason it's jarring int.hex() doesn't exist.

If Mark or I wanted to use float._as_integer_ratio() directly too, that's fine: we're numeric grownups and won't throw a hissy fit if ints don't support it too ;-)

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