argparse: allow to override the type name in `'invalid %(type)s value: %(value)r'` error messages

[calestyo]

Feature or enhancement

Proposal:

Hey.

When doing something like:

class SomeEnumClass(enum.StrEnum):
    foo = enum.auto()
    bar = enum.auto()

parser.add_argument("--enum", type=SomeEnumClass, choices=[e.value  for e in SomeEnumClass])

and calls --enum with argument baz, it will error out with a message like:

my.py: error: argument --enum: invalid SomeEnumClass value: 'baz'

via:

cpython/Lib/argparse.py

Line 2540 in d0b92dd

msg = _('invalid %(type)s value: %(value)r')

For the end user (who sees this message), the SomeEnumClass-part may be rather useless, as it may be some arbitrary internal class name, which may or may not be related to the option.

So in principle I think one should be able to configure a string to be printed instead of SomeEnumClass.

One candidate would be of course metavar, but the problem with that is, that if one sets it, the --help-text would be different, too, and not longer print the choices, but rather the metavar, which one may not want.

The only way I've found to get that without extending argparse is:

class SomeEnumClass(enum.StrEnum):
    foo = enum.auto()
    bar = enum.auto()
SomeEnumClass.__name__ = "blubb"

which causes:

my.py: error: argument --enum: invalid blubb value: 'baz'

to be printed,... but I guess is rather not really well defined, or is it?

Cheers,
Chris.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• GH-122679: Add register() to argparse docs #126939
• [3.12] GH-122679: Add register() to argparse docs (GH-126939) #127148
• [3.13] GH-122679: Add register() to argparse docs (GH-126939) #127149

[serhiy-storchaka]

You can register the type and specify it by name.

parser.register('type', 'blubb', SomeEnumClass)
parser.add_argument("--enum", type='blubb', choices=[e.value  for e in SomeEnumClass])

[calestyo]

Nice... that should be added to the argparse documentation.