Step 1: Provide a summary of your problem

On Python 3.15.0, every tmuxp invocation, including tmuxp --version, fails with AttributeError: 'NoneType' object has no attribute 'reset' while the argument parser is being built. TmuxpHelpFormatter declares a class attribute _theme that shadows the _theme property of argparse.HelpFormatter, which 3.15 added to support coloured help. Much of the test suite fails for the same reason; it passes on Python 3.14.

Step 2: Provide tmuxp details

tmuxp debug-info crashes on 3.15 with the same error, so these versions come from the checkout and uv.lock:

• tmuxp 1.74.0 at ddd6cce, libtmux 0.61.0 (from uv.lock)
• Python 3.15.0
• uv 0.13.1
• Linux x86_64

Step 3: Describe the problem:

Steps to reproduce:

1. Check out the repository and install the project with all extras and dev dependencies on Python 3.15.

$ uv sync --python 3.15 --all-extras --dev

2. Run any command.

$ uv run tmuxp --version

3. Or run a single test.

$ uv run py.test tests/cli/test_cli.py -k help_long_flag

Observed Results:

AttributeError: 'NoneType' object has no attribute 'reset'

Without colour (no TTY, or NO_COLOR set), create_themed_formatter() binds theme = None, so argparse reads None for self._theme in _apply_text_markup. With colour forced (FORCE_COLOR=1), the bound HelpTheme is read instead and fails with AttributeError: 'HelpTheme' object has no attribute 'prog_extra', because argparse expects its own theme object.

Traceback (most recent call last):
  File "<repo>/.venv/bin/tmuxp", line 10, in <module>
    sys.exit(cli.cli())
             ~~~~~~~^^
  File "<repo>/src/tmuxp/cli/__init__.py", line 297, in cli
    parser = create_parser()
  File "<repo>/src/tmuxp/cli/__init__.py", line 169, in create_parser
    parser = argparse.ArgumentParser(
        prog="tmuxp",
        description=CLI_DESCRIPTION,
        formatter_class=formatter_class,
    )
  File "<stdlib>/argparse.py", line 2058, in __init__
    self.add_argument(
  File "<stdlib>/argparse.py", line 1695, in add_argument
    self._check_help(action)
  File "<stdlib>/argparse.py", line 1887, in _check_help
    formatter._expand_help(action)
  File "<stdlib>/argparse.py", line 716, in _expand_help
    return self._apply_text_markup(help_string)
  File "<stdlib>/argparse.py", line 568, in _apply_text_markup
    if not t.reset:
           ^^^^^^^
AttributeError: 'NoneType' object has no attribute 'reset'

Expected Results:

tmuxp --version prints the version, and the test suite passes as it does on Python 3.14.

Relevant Code:

Python 3.15 added backtick markup to argparse help text (What's New in Python 3.15, python/cpython#142389, python/cpython#149375). argparse.HelpFormatter now computes its colours through a read-only _theme property and reads it in _apply_text_markup. The excerpt is from Lib/argparse.py at v3.15.0 and lines 559-570:

    @property
    def _theme(self):
        if self._cached_theme is None:
            self._get_theme_and_decolor()
        return self._cached_theme

    def _apply_text_markup(self, text):
        ...
        t = self._theme
        if not t.reset:
            return text

tmuxp's formatter already used _theme for its own HelpTheme, so it now collides with the stdlib name: the class attribute at src/tmuxp/cli/_formatter.py lines 85-86 replaces the property, and the instance assignment in lines 391-396 sets the value argparse then reads. The parser is built at src/tmuxp/cli/__init__.py lines 163-173, and ArgumentParser.__init__ adds -h/--help, whose help text goes through _apply_text_markup immediately.

Proposal

Store tmuxp's theme under a name argparse does not use, for example _help_theme, in TmuxpHelpFormatter._fill_text, _colorize_example_line, and create_themed_formatter, and leave _theme to the standard library.

References

• What's New in Python 3.15
• argparse documentation for Python 3.15
• python/cpython#142389: backtick colourisation in argparse help text
• python/cpython#149375: backtick markup in argument help