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Cannot use format specifiers in argparse help #142950

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Cannot use format specifiers in argparse help#142950
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savannahostrowski
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3.15pre-release feature fixes, bugs and security fixespre-release feature fixes, bugs and security fixesstdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error
@savannahostrowski

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@savannahostrowski
savannahostrowski
opened on Dec 18, 2025
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#141940 caused a regression where users cannot use format specifiers.

See this as an example

my_parser.add_argument(
        "--foo",
        type=int,
        default=1234,
        help=f"""0x%(default)x""",
    )

The above works in 3.14.2, but not in 3.15 alpha3. I apologize if my code is doing something incorrect.

The error I get is

Traceback (most recent call last):
  File "[...]/3.15.0a3/lib/python3.15/argparse.py", line 1793, in _check_help
    formatter._expand_help(action)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "[...]/3.15.0a3/lib/python3.15/argparse.py", line 696, in _expand_help
    return help_string % params
           ~~~~~~~~~~~~^~~~~~~~
TypeError: %x format: an integer is required, not str

Originally posted by @oyvindronningstad in #141940 (comment)

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3.15pre-release feature fixes, bugs and security fixespre-release feature fixes, bugs and security fixesstdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

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