gh-67790: Support float-style formatting for Fraction instances

[mdickinson]

This PR adds support for float-style formatting for Fraction objects: it supports the "e", "E", "f", "F", "g", "G" and "%" presentation types, and all the various bells and whistles of the formatting mini-language for those presentation types. The behaviour almost exactly matches that of float, but the implementation works with the exact Fraction value and does not do an intermediate conversion to float, and so avoids loss of precision or issues with numbers that are outside the dynamic range of the float type.

Note that the "n" presentation type is not supported. That support could be added later if people have a need for it.

There's one corner-case where the behaviour differs from that of float: for the float type, if explicit alignment is specified with a fill character of '0' and alignment type '=', then thousands separators (if specified) are inserted into the padding string:

>>> format(3.14, '0=11,.2f')
'0,000,003.14'

The exact same effect can be achieved by using the '0' flag:

>>> format(3.14, '011,.2f')
'0,000,003.14'

For Fraction, only the '0' flag has the above behaviour with respect to thousands separators: there's no special-casing of the particular '0=' fill-character/alignment combination. Instead, we treat the fill character '0' just like any other:

>>> format(Fraction('3.14'), '0=11,.2f')
'00000003.14'
>>> format(Fraction('3.14'), '011,.2f')
'0,000,003.14'

The Fraction formatter is also stricter about combining these two things: it's not permitted to use both the '0' flag and explicit alignment, on the basis that we should refuse the temptation to guess in the face of ambiguity. float is less picky:

>>> format(3.14, '0<011,.2f')
'3.140000000'
>>> format(Fraction('3.14'), '0<011,.2f')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Repositories/python/cpython/Lib/fractions.py", line 414, in __format__
    raise ValueError(
ValueError: Invalid format specifier '0<011,.2f' for object of type 'Fraction'; can't use explicit alignment when zero-padding

• Issue: Implement __format__ for Fraction #67790

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[mdickinson]

Converting to draft while I pacify the doc build.

[mdickinson]

Doc build duly pacified; ready for review.

[mdickinson]

@ericvsmith Would you be willing to review at some point? I'm not looking for detailed line-by-line review (though that would be useful too) so much as big-picture "is this a good idea?" review. In particular, I want to avoid doing anything here that will be hard to undo later if it conflicts with a different approach that we want to take, and that's why I restricted to just implementing the efg% presentation types, where the desired behaviour seems reasonably clear.

[ericvsmith]

Hi, @mdickinson. Yes, I'll take a look. I'm going to be out of town for a few days, but will review when I get back.

[ericvsmith]

I forgot to add: yes, I think this is a good idea!

[mdickinson]

@ericvsmith Thanks for reviewing! I'll do another round of self-review, and merge shortly if I don't spot anything blocking.

So I guess as long as we reserve the integer presentation types ('b', 'd', 'o', 'x', 'X'), nothing precludes them being used in the future.

Yes, that was where I was going. Of those, I'm finding it hard to imagine use-cases for anything other than presentation type 'd', though. If I have time before Python 3.12, I'd like to prepare a follow-up PR for presentation type 'd' that:

• with no other information, behaves identically to str
• supports fill (with the minimum length applied to the whole output string), alignment, sign manipulation and thousands separators (with the latter applied to both numerator and denominator, as you suggest)
• does not support zero-fill, because it's not clear exactly what that means
• makes 'd' the "default" presentation type, so that the presentation type itself can be omitted.
• as a bonus, supports the '#' "alternate" flag to mean "always express the result as a fraction, even it it's an integer". So e.g., Fraction(3) would be presented as 3/1 rather than just 3. This seems consistent with the uses of '#' in float-style formatting to describe a format that strictly conforms to specifications (keeping a decimal point and trailing zeros) as opposed to being human-friendly (stripping trailing zeros and point).

[mdickinson]

the formatting would be applied to both the numerator and denominator, and weird things happen with the width [...]

I think I'm missing something here. Precision doesn't apply, of course, so it's only the width we need to worry about. I was imagining that this would simply be a minimum width, applied to the formatted fraction as a whole. So for example:

>>> f"{Fraction(22, 7):}"
'22/7'
>>> f"{Fraction(22, 7):10}"  # or '10d' in place of '10'
'      22/7'

I guess that would mean that the slashes wouldn't be nicely aligned in a table of fractions, but I think I could live with that - if anyone needs that alignment they could format the numerator and denominator separately:

>>> f = Fraction(22, 7)
>>> f"{f.numerator:>5}/{f.denominator:<5}"
'   22/7    '

We could try to get fancy and re-use the precision as a minimum width for the denominator:

>>> f"{Fraction(22, 7):10.5}"
'  22/    7'

But this feels like a bit of an abuse, and it also feels as though we're getting into YAGNI territory, and that we're guessing about use-cases without information from actual users.

[mdickinson]

@ericvsmith I've made a proof-of-concept PR (against the branch for this PR) for adding support for the 'd' presentation type at mdickinson#2.

[ericvsmith]

I think I'm missing something here. Precision doesn't apply, of course, so it's only the width we need to worry about. I was imagining that this would simply be a minimum width, applied to the formatted fraction as a whole.

I think you're right in applying the width to the whole fraction, including the slash. No need to go nutso on this, and as you say, the users could format the numerator and denominator separately. I'll take a look at your branch.

[mdickinson]

@ericvsmith Thanks.

I'll merge this one. There were a few inconsequential style / cosmetic updates since your review, and one bugfix + regression test ("Z" was being accepted as the suppress-negative-zero flag, where only "z" should have been allowed).

[bszonye]

I guess that would mean that the slashes wouldn't be nicely aligned in a table of fractions, but I think I could live with that - if anyone needs that alignment they could format the numerator and denominator separately....

For what it's worth, I spent about a day implementing a tabular format for fractions that aligned the slashes, and I threw the whole thing out because left-aligning the denominators made the fractions too hard to read. Alignment around a radix point works great because it keeps all of the tens, units, tenths, etc. in vertical alignment. The same isn't true for fraction slashes (unless you normalize everything to the same denominator) so it just makes the rows ragged and harder to scan.