mypy crashes with python 3.14 within argparse

[kasium]

Crash Report

When using python 3.14, mypy crashes right away. This is due the AugmentedHelpFormatter class in mypy which does not accept all arguments of argparse.HelpFormatter. In python 3.14,_get_formatter of argparse will pass prefix_chars which is not supported by AugmentedHelpFormatter.

Proposal: change the constructor of AugmentedHelpFormatter like

class AugmentedHelpFormatter(argparse.RawDescriptionHelpFormatter):
 def __init__(self, prog: str, **kwargs: Any) -> None:
 super().__init__(prog=prog, max_help_position=28, **kwargs)

By the way, the same applies to the color keyword, which is passed as well

Traceback

 File "venv314/lib/python3.14/site-packages/mypy/__main__.py", line 15, in console_entry
 main()
 ~~~~^^
 File "venv314/lib/python3.14/site-packages/mypy/main.py", line 88, in main
 sources, options = process_options(args, stdout=stdout, stderr=stderr, fscache=fscache)
 ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "venv314/lib/python3.14/site-packages/mypy/main.py", line 1055, in process_options
 parser.add_argument(
 ~~~~~~~~~~~~~~~~~~~^
 "--enable-incomplete-feature",
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...<2 lines>...
 help="Enable support of incomplete/experimental features for early preview",
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 )
 ^
 File "/data/.pyenv/versions/3.14.0b1/lib/python3.14/argparse.py", line 1562, in add_argument
 formatter = self._get_formatter()
 File "/data/.pyenv/versions/3.14.0b1/lib/python3.14/argparse.py", line 2729, in _get_formatter
 return self.formatter_class(
 ~~~~~~~~~~~~~~~~~~~~^
 prog=self.prog,
 ^^^^^^^^^^^^^^^
 prefix_chars=self.prefix_chars,
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 color=self.color,
 ^^^^^^^^^^^^^^^^^
 )
 ^
TypeError: AugmentedHelpFormatter.__init__() got an unexpected keyword argument 'prefix_chars'.)

To Reproduce

1. setup a python 3.14 environment
2. run mypy --version

Your Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: --version
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.1.4.0b1
• Operating system and version: Linux

[A5rocks]

This should be fixed by #19020 which should be in mypy 1.16

[kasium]

Thanks a lot!

[brianschubert]

Note for posterity: this was also patched upstream for 3.14.0b2+

python/cpython#133653