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GH-142950: Process format specifiers before colourization in argparse help #142960

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GH-142950: Process format specifiers before colourization in argparse help #142960

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Format before colourization
savannahostrowski Dec 18, 2025
987af8b
Clean up
savannahostrowski Dec 18, 2025
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savannahostrowski Dec 18, 2025
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📜🤖 Added by blurb_it.
blurb-it[bot] Dec 18, 2025
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savannahostrowski Dec 18, 2025
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26 changes: 22 additions & 4 deletions Lib/argparse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -688,11 +688,29 @@ def _expand_help(self, action):
params[name] = value.__name__
if params.get('choices') is not None:
params['choices'] = ', '.join(map(str, params['choices']))
# Before interpolating, wrap the values with color codes

t = self._theme
for name, value in params.items():
params[name] = f"{t.interpolated_value}{value}{t.reset}"
return help_string % params

if not t.reset:
return help_string % params

# Format first to preserve types for specifiers, like %x that require int.
def colorize(match):
spec, name = match.group(0, 1)
if spec == '%%':
return '%'
if name in params:
formatted = spec % {name: params[name]}
return f'{t.interpolated_value}{formatted}{t.reset}'
return spec

# Match %% or %(name)... format specifiers
result = _re.sub(r'%%|%\((\w+)\)[^a-z]*[a-z]', colorize,
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@serhiy-storchaka serhiy-storchaka Dec 22, 2025

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Any characters can occur in the key name, not only \w. It is even more complicated, because nested ( and ) should be balanced, but this is impossible to check with a regular expression.

There are formatting codes denoted by the upper case: "X", "E", "F" and "G".

Some letters before the format specifier should be ignored: "h", "l" and "L".

help_string, flags=_re.IGNORECASE)

if '%' in result:
raise ValueError(f"invalid format specifier in: {help_string!r}")
return result
Comment on lines +693 to +713
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@hamdanal hamdanal Dec 20, 2025

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I also hit the issue linked here in my nightly tests.
From a look at this fix, it looks like it would not handle all edge cases of % formatting. In addition to the case mentioned above #142960 (comment), the regex used here doesn't handle length modifiers. Here is a modified patch that handles both issues and uses a regex that emulated Python's % formatting as described here

Suggested change
if not t.reset:
return help_string % params
# Format first to preserve types for specifiers, like %x that require int.
def colorize(match):
spec, name = match.group(0, 1)
if spec == '%%':
return '%'
if name in params:
formatted = spec % {name: params[name]}
return f'{t.interpolated_value}{formatted}{t.reset}'
return spec
# Match %% or %(name)... format specifiers
result = _re.sub(r'%%|%\((\w+)\)[^a-z]*[a-z]', colorize,
help_string, flags=_re.IGNORECASE)
if '%' in result:
raise ValueError(f"invalid format specifier in: {help_string!r}")
return result
result = help_string % params # always format the whole string to raise on invalid input
if not t.reset:
return result
# Format first to preserve types for specifiers, like %x that require int.
def colorize(match):
spec, name = match.group(0, 'mapping')
if spec == '%%':
return '%'
if name in params:
formatted = spec % {name: params[name]}
return f'{t.interpolated_value}{formatted}{t.reset}'
return spec
# Match %% or %(name)... format specifiers
result = _re.sub(
r"""
% # Percent character
(?:\((?P<mapping>[^)]*)\))? # Mapping key
(?P<flag>[#0\-+ ])? # Conversion Flags
(?P<width>\*|\d+)? # Minimum field width
(?P<precision>\.(?:\*?|\d*))? # Precision
[hlL]? # Length modifier (ignored)
(?P<format>[diouxXeEfFgGcrsa%]) # Conversion type
""",
colorize,
help_string,
flags=_re.VERBOSE,
)
return result


def _iter_indented_subactions(self, action):
try:
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16 changes: 16 additions & 0 deletions Lib/test/test_argparse.py
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Original file line number Diff line number Diff line change
Expand Up @@ -7663,6 +7663,22 @@ def test_backtick_markup_special_regex_chars(self):
help_text = parser.format_help()
self.assertIn(f'{prog_extra}grep "foo.*bar" | sort{reset}', help_text)

def test_help_with_format_specifiers(self):
# GH-142950: format specifiers like %x should work with color=True
parser = argparse.ArgumentParser(prog='PROG', color=True)
parser.add_argument('--hex', type=int, default=255,
help='hex: %(default)x')
parser.add_argument('--str', default='test',
help='str: %(default)s')

help_text = parser.format_help()

interp = self.theme.interpolated_value
reset = self.theme.reset

self.assertIn(f'hex: {interp}ff{reset}', help_text)
self.assertIn(f'str: {interp}test{reset}', help_text)

def test_print_help_uses_target_file_for_color_decision(self):
parser = argparse.ArgumentParser(prog='PROG', color=True)
parser.add_argument('--opt')
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2025-12-18-22-58-46.gh-issue-142950.EJ8w-T.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix regression in :mod:`argparse` where format specifiers in help strings raised :exc:`ValueError`.
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