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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Cannot use lazy translation proxies in argparse help","articleBody":"# Bug report\n\n### Bug description:\n\nhttps://github.com/python/cpython/pull/142960 introduced a regression for lazy strings in the `help=` parameter of `ArgumentParser.add_argument()`. This is used at least [by the Sphinx CLI](https://github.com/sphinx-doc/sphinx/blob/cc7c6f435ad37bb12264f8118c8461b230e6830c/sphinx/ext/autosummary/generate.py#L898-L900) with its [`_TranslationProxy`](https://github.com/sphinx-doc/sphinx/blob/cc7c6f435ad37bb12264f8118c8461b230e6830c/sphinx/locale/__init__.py#L21-L26), at least during testing, where [`sphinx.locale.init_console` is monkeypatched](https://github.com/sphinx-doc/sphinx/blob/cc7c6f435ad37bb12264f8118c8461b230e6830c/tests/conftest.py#L39).\n\nThe new version of `_expand_help()` calls `re.sub()` on help strings when color is enabled, but `re.sub()` requires an actual str/bytes object and rejects string-like proxies that only implement `__str__`/`__mod__`.\n\n```python\n# mre_argparse_lazy_str.py\n\nimport argparse\n\n\nclass LazyStr:\n    \"\"\"Minimal stand-in for gettext lazy-translation objects.\"\"\"\n\n    def __init__(self, message: str) -\u003e None:\n        self._message = message\n\n    def __str__(self) -\u003e str:\n        return self._message  # resolve lazily\n\n    def __mod__(self, other):\n        return self._message % other  # supports 'help % params'\n\n    def __contains__(self, item) -\u003e bool:\n        return item in self._message  # supports '\"%\" in help'\n\n\np = argparse.ArgumentParser(description='demo')\n# Works fine without color; fails with FORCE_COLOR=1 (or a color TTY):\np.add_argument('--output', default='out.txt', help=LazyStr('write to %(default)s'))\np.parse_args([])\nprint('OK – parsed without error')\n```\n\n```console\n$ uvx -p 3.15.0a4 python mre_argparse_lazy_str.py\nOK – parsed without error\n```\n\n```console\n$ uvx -p 3.15.0a5 python mre_argparse_lazy_str.py\nTraceback (most recent call last):\n  File \"/Users/personal/.local/share/uv/python/cpython-3.15.0a5-macos-aarch64-none/lib/python3.15/argparse.py\", line 1822, in _check_help\n    formatter._expand_help(action)\n    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^\n  File \"/Users/personal/.local/share/uv/python/cpython-3.15.0a5-macos-aarch64-none/lib/python3.15/argparse.py\", line 725, in _expand_help\n    return _re.sub(fmt_spec, colorize, help_string, flags=_re.VERBOSE)\n           ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/personal/.local/share/uv/python/cpython-3.15.0a5-macos-aarch64-none/lib/python3.15/re/__init__.py\", line 206, in sub\n    return _compile(pattern, flags).sub(repl, string, count)\n           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^\nTypeError: expected string or bytes-like object, got 'LazyStr'\n\nThe above exception was the direct cause of the following exception:\n\nTraceback (most recent call last):\n  File \"/Users/personal/Contrib/sphinx-doc/sphinx/mre_argparse_lazy_str.py\", line 47, in \u003cmodule\u003e\n    p.add_argument('--output', default='out.txt', help=LazyStr('write to %(default)s'))\n    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/personal/.local/share/uv/python/cpython-3.15.0a5-macos-aarch64-none/lib/python3.15/argparse.py\", line 1630, in add_argument\n    self._check_help(action)\n    ~~~~~~~~~~~~~~~~^^^^^^^^\n  File \"/Users/personal/.local/share/uv/python/cpython-3.15.0a5-macos-aarch64-none/lib/python3.15/argparse.py\", line 1824, in _check_help\n    raise ValueError('badly formed help string') from exc\nValueError: badly formed help string\n```\n\nIf this indeed something CPython rather than Sphinx should fix, the simplest solution would be to coerce `help_string`:\n\n```diff\n- return _re.sub(fmt_spec, colorize, help_string, flags=_re.VERBOSE)\n+ return _re.sub(fmt_spec, colorize, str(help_string), flags=_re.VERBOSE)\n```\n\nand I'd be happy to send a PR if that's the case.\n\n## Related\n\n- https://github.com/python/cpython/issues/142950\n\n### CPython versions tested on:\n\n3.15\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-150276\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/edgarrmondragon","@type":"Person","name":"edgarrmondragon"},"datePublished":"2026-04-12T19:42:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/148468/cpython/issues/148468"}