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