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.

+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.

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...

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

> 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.

New changeset 89e50ab36fac6a0e7f1998501f36fcd2872a6604 by Sergey B Kirpichev in branch 'main':
bpo-44258: support PEP 515 for Fraction's initialization from string (GH-26422)
https://github.com/python/cpython/commit/89e50ab36fac6a0e7f1998501f36fcd2872a6604