Python 3.14: TypeError: HelpFormatter.__init__() got an unexpected keyword argument 'prefix_chars'

[The-Compiler]

Issue

With Python 3.14.0b1, tox immediately fails to start with TypeError: HelpFormatter.__init__() got an unexpected keyword argument 'prefix_chars', because it overrides HelpFormatter in an incompatible way.

This is due to:

• gh-130645: Add colour to argparse help python/cpython#132323

See also:

• Fix HelpFormatter for Python 3.14 pypa/virtualenv#2878
• Fix argparse for Python 3.14 python/mypy#19020

Environment

Provide at least:

• OS: Archlinux, Python 3.14.0b1

Output of pip list of the host Python, where tox is installed

cachetools    5.5.2
chardet       5.2.0
colorama      0.4.6
distlib       0.3.9
filelock      3.18.0
packaging     25.0
pip           25.1.1
platformdirs  4.3.8
pluggy        1.5.0
pyproject-api 1.9.0
tox           4.25.1.dev6+g4a8e50e2 /home/florian/tmp/tox
virtualenv    20.31.1

Output of running tox

Output of tox -rvv

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/florian/tmp/tox/src/tox/__main__.py", line 6, in <module>
    run()
    ~~~^^
  File "/home/florian/tmp/tox/src/tox/run.py", line 20, in run
    result = main(sys.argv[1:] if args is None else args)
  File "/home/florian/tmp/tox/src/tox/run.py", line 39, in main
    state = setup_state(args)
  File "/home/florian/tmp/tox/src/tox/run.py", line 53, in setup_state
    options = get_options(*args)
  File "/home/florian/tmp/tox/src/tox/config/cli/parse.py", line 39, in get_options
    parsed, cmd_handlers = _get_all(args)
                           ~~~~~~~~^^^^^^
  File "/home/florian/tmp/tox/src/tox/config/cli/parse.py", line 69, in _get_all
    tox_parser = _get_parser()
  File "/home/florian/tmp/tox/src/tox/config/cli/parse.py", line 76, in _get_parser
    tox_parser = ToxParser.core()  # load the core options
  File "/home/florian/tmp/tox/src/tox/config/cli/parser.py", line 300, in core
    return cls(
        prog=NAME,
    ...<3 lines>...
        description="create and set up environments to run command(s) in them",
    )
  File "/home/florian/tmp/tox/src/tox/config/cli/parser.py", line 149, in __init__
    super().__init__(*args, **kwargs)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/florian/tmp/tox/src/tox/config/cli/parser.py", line 39, in __init__
    super().__init__(*args, **kwargs)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/argparse.py", line 1920, in __init__
    self.add_argument(
    ~~~~~~~~~~~~~~~~~^
        default_prefix+'h', default_prefix*2+'help',
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        action='help', default=SUPPRESS,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        help=_('show this help message and exit'))
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/florian/tmp/tox/src/tox/config/cli/parser.py", line 284, in add_argument
    result = super().add_argument(*args, **kwargs)
  File "/usr/lib/python3.14/argparse.py", line 1562, in add_argument
    formatter = self._get_formatter()
  File "/usr/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: HelpFormatter.__init__() got an unexpected keyword argument 'prefix_chars'

[hugovk]

We'll also fix this in CPython for beta 2 (python/cpython#133653), but it's also helpful to if tox makes a (temporary or not) fix to enable testing beta 1, thanks!

[hroncok]

See also pypa/virtualenv#2878 which fixes the same in virtualenv.