Support PEP 515 for Fraction's initialization from string

[skirpichev]

BPO	44258
Nosy	@mdickinson, @vedgar, @skirpichev
PRs	bpo-44258: support PEP 515 for Fraction's initialization from string #26422
Files	fractions-from-str.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-06-07.07:07:33.971>
created_at = <Date 2021-05-28.05:47:31.699>
labels = ['type-feature', 'library', '3.11']
title = "Support PEP 515 for Fraction's initialization from string"
updated_at = <Date 2021-06-07.07:07:33.970>
user = 'https://github.com/skirpichev'

bugs.python.org fields:

activity = <Date 2021-06-07.07:07:33.970>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-07.07:07:33.971>
closer = 'mark.dickinson'
components = ['Library (Lib)']
creation = <Date 2021-05-28.05:47:31.699>
creator = 'Sergey.Kirpichev'
dependencies = []
files = ['50069']
hgrepos = []
issue_num = 44258
keywords = ['patch']
message_count = 6.0
messages = ['394633', '394638', '394642', '394730', '394731', '395249']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'veky', 'Sergey.Kirpichev']
pr_nums = ['26422']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44258'
versions = ['Python 3.11']

[skirpichev]

Right now:
>>> from fractions import Fraction as F
>>> F(1_2_3, 3_2_1)
Fraction(41, 107)

but

>>> F('1_2_3/3_2_1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sk/src/cpython/Lib/fractions.py", line 115, in __new__
    raise ValueError('Invalid literal for Fraction: %r' %
ValueError: Invalid literal for Fraction: '1_2_3/3_2_1'

or even this (should be consistent with int constructor, isn't?):
>>> F('1_2_3')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sk/src/cpython/Lib/fractions.py", line 115, in __new__
    raise ValueError('Invalid literal for Fraction: %r' %
ValueError: Invalid literal for Fraction: '1_2_3'

Tentative patch attached. Let me know if this does make sense as a PR.

[mdickinson]

+1 to the idea: Decimal, float, complex and int all understand the underscores; there's no good reason for Fraction not to.

So yes please to the PR. There should be tests that check that only the underscore rules allowed by int and the others are supported: e.g., Fraction("1__2"), and Fraction("1_2_") should be errors.

[skirpichev]

On Fri, May 28, 2021 at 06:48:14AM +0000, Mark Dickinson wrote:

So yes please to the PR. There should be tests that check that only the
underscore rules allowed by int and the others are supported: e.g.,
Fraction("1__2"), and Fraction("1_2_") should be errors.

Ok, I did.

In the initial version I catch int()'s exceptions to
return a correct exception details. Maybe it's better to fix
the regexp instead (don't match "wrong" strings). I think
this might be more complicated for readers...

[vedgar]

How about '1_/_2'? I think making / more separated adds value... though of course, someone will ask about '1 / 2' next. :-)

[mdickinson]

How about '1_/_2'?

I'd rather keep it consistent with the rules for int: that is, if I split on '/', I'd expect the pieces to be parseable by int.

As for spaces around the /, let's make that a separate issue.

[mdickinson]

New changeset 89e50ab by Sergey B Kirpichev in branch 'main':
bpo-44258: support PEP-515 for Fraction's initialization from string (GH-26422)
89e50ab