The form of format specifier is the following

[[fill]align][sign]["z"]["#"]["0"][width][grouping_option]["." precision][type]

Docs describe each option in the following order:

1. align and fill.
2. sign.
3. Option z.
4. Option #.
5. Option ,.
6. Option _.
7. width, then option 0.
8. precision.
9. type.

As you can notice, options , and _ are not present in format spec, because they are actually part of grouping_option. For whatever reason they are described before width, but most importantly, their descriptions never mention they are part of grouping_option.

It can be super confusing for the first readthrough, as descriptions are out of order and written similar to standalone options z and #. I suggest the following changes:

1. Place descriptions of options , and _ after the description of width.
2. Mention that they are part of grouping_option.
3. Maybe change BNF specification of grouping_option to "," | "_" to match their description order and order of addition to Python.

Example change:

The grouping_option determines how digits in numbers are grouped. Its first option, ',', signals...

The other grouping_option, '_', signals...

Alternatively, these description can be placed in a table, similar for the list of string presentation types.

The grouping_option determines how digits in numbers are grouped. It can be one of the following:

==here goes the table==

Linked PRs

• gh-131912: Refactored description of grouping options in format specification docs #132030
• [3.13] gh-131912: Refactor description of grouping options in format specification docs (GH-132030) #132123
• [3.12] gh-131912: Refactor description of grouping options in format specification docs (GH-132030) #132203
• gh-131912: Use different grouping options for the integral and fractional parts #132170