Just want to share that the new errors seems a bit cryptic. In pyperformance, we had argparsers suddenly start failing with:

Traceback (most recent call last):
  File "/home/benchmarking/actions-runner/_work/benchmarking/benchmarking/cpython/Lib/argparse.py", line 1659, in _check_help
    formatter._expand_help(action)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/home/benchmarking/actions-runner/_work/benchmarking/benchmarking/cpython/Lib/argparse.py", line 604, in _expand_help
    return help_string % params
           ~~~~~~~~~~~~^~~~~~~~
TypeError: %o format: an integer is required, not dict

The issue was that the help string for one argument contained the phrase 90% of, which is super subtle. I wonder if maybe a better error message could be produced here?

This is not specific to arparse. You get the same error message with pure printf-style error formatting:

>>> '90% of' % {'a': 1}
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    '90% of' % {'a': 1}
    ~~~~~~~~~^~~~~~~~~~
TypeError: %o format: an integer is required, not dict

Please open a new issue if you have ideas how to improve this.

In argparse, the full traceback refers also to the line in your code that adds an invalid help string, so get a guide where to search for error.