Preceding the width field by `'0'` affects the default alignment for strings using `str.format()` on Python 3.12.7

[hyperkai]

Bug report

Bug description:

[The doc](https://docs.python.org/3/library/string.html#format-specification-mini-language) says below:

Changed in version 3.10: Preceding the width field by '0' no longer affects the default alignment for strings.

But, preceding the width field by '0' affects the default alignment for strings using str.format() says below:
on Python 3.12.7 as shown below:

v = "Hello"

#          ↓
print('{0:<08}'.format(v))
# Hello000

#          ↓
print('{0:>08}'.format(v))
# 000Hello

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[skirpichev]

preceding the width field by '0' affects the default alignment for strings
print('{0:<08}'.format(v))

It's not true. The grammar says:

format_spec ::= [options][width][grouping]["." precision][type]
options ::= [[fill]align][sign]["z"]["#"]["0"]

That means, that fill is optional if alignment is specified (or the default is space). So, 0 in your example is a part of the width field.

But you can see the difference, mentioned in docs:

Python 3.9.22 (tags/v3.9.22:05c5549224, Apr  9 2025, 06:33:10) 
[GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> '{0:08}'.format("Hello")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier

Python 3.10.17 (tags/v3.10.17:26ee8cad13, Apr  9 2025, 09:05:04) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> '{0:08}'.format("Hello")
'Hello000'