[Bug]: invalid handling of digit group separators in formatting non-finite floats

[skirpichev]

Describe the bug

This broken when sign-aware zero-padding enabled:

Python 3.12.8 (Tue Jun 23 15:26:01 UTC 2026)
[Graal, Oracle GraalVM, Java 25.0.3 (amd64)] on 'linux'
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float('inf'), '07,f')
'0,000inf'

c.f.

Python 3.12.13 (tags/v3.12.13:3bb231a6a5d, May 14 2026, 11:06:30) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format(float('inf'), '07,f')
'0000inf'

Perhaps, documentation is more vague here, but:

The grouping option after the width field specifies a digit group separator for the integral part of a number.

Neither nan, nor infinities have "integral part"...

Operating system

Linux

CPU architecture

x86_64

GraalPy version

25.1.3

JDK version

No response

Context configuration

No response

Steps to reproduce

See description

Expected behavior

As in CPython 3.12

Stack trace

Additional context

No response