>>> import argparse
>>> 
>>> parser = argparse.ArgumentParser()
>>> _ = parser.add_argument('foo', choices=[], help='%(choices)s')
>>> parser.print_help()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 2358, in print_help
    self._print_message(self.format_help(), file)
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 2342, in format_help
    return formatter.format_help()
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 278, in format_help
    help = self._root_section.format_help()
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 208, in format_help
    func(*args)
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 208, in format_help
    func(*args)
  File "/home/guest/tmp/tests/misc/git/example/cpython/main/Lib/argparse.py", line 517, in _format_action
    parts.append('%*s%s\n' % (indent_first, '', help_lines[0]))
IndexError: list index out of range
>>>

It's not very useful to print empty choices, but the choices list could be formed dynamically. So the command-line user can't figure out what's happen.

Does this work for

    help='choices: %(choices)s'

Without setting up my own test case, I'm guessing that the problem is with an empty help string.  `help=''` should be ok, but the test handle that probably occurs earlier.

Choices, empty or not, are included earlier in the help line, as well as usage.  So there isn't real need to include them in the user defined part of the help line.

Still, there are other bug issues regarding the formatting of choices.  That part needs refinement to handle very long lists of choices, as well as ones that don't fit the usual 'list' format.

This error is also produced by

    help = ' '

that is, a help line with all blanks.  As long as there is a nonblank character in the help line it format fine.

This issue doesn't need to be resolved by itself, but should be considered if and when the formatting of choices is reworked.

New changeset 6fafc25aea8689048314b5bf7a9bb986bb1ce238 by andrei kulakov in branch 'main':
bpo-24444: fix an error in argparse help when help for an option is blank (GH-28050)
https://github.com/python/cpython/commit/6fafc25aea8689048314b5bf7a9bb986bb1ce238

New changeset fd2be6da2ffec2301a58aefa92f17f0c567fbc55 by Miss Islington (bot) in branch '3.10':
bpo-24444: fix an error in argparse help when help for an option is blank (GH-28050) (GH-28931)
https://github.com/python/cpython/commit/fd2be6da2ffec2301a58aefa92f17f0c567fbc55

New changeset fb7203453855a43907048ff2b8f9b890305b3d15 by Miss Islington (bot) in branch '3.9':
[3.9] bpo-24444: fix an error in argparse help when help for an option is blank (GH-28050) (GH-28932)
https://github.com/python/cpython/commit/fb7203453855a43907048ff2b8f9b890305b3d15

Thanks, Andrei! ✨ 🍰 ✨