Cannot use format specifiers in argparse help

[savannahostrowski]

#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)

Linked PRs

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

[gitlostmyaccounttt]

This is a regression, and your report is valid.

help=f"""0x%(default)x""" this is an f-string but it doesn't contain {},

it evaluates to the literal string "0x%(default)x". argparse later does:

help_string % params

This is a regression caused by #141940, just like you said

Try to remove %x and format yourself like

my_parser.add_argument(`
 "--foo",
 type=int,
 default=1234,
 help=f"0x{1234:x}",
)

You did not misuse argparse, this is a regression like you said. Don't depend on % expansion for numeric formatting