python – Argparse: Way to include default values in ‘–help’?

Suppose I have the following argparse snippet:

diags.cmdln_parser.add_argument( '--scan-time',
                     action  = 'store',
                     nargs   = '?',
                     type    = int,
                     default = 5,
                     help    = "Wait SCAN-TIME seconds between status checks.")

Currently, --help returns:

usage: connection_check.py [-h]
                             [--version] [--scan-time [SCAN_TIME]]

          Test the reliability/uptime of a connection.



optional arguments:
-h, --help            show this help message and exit
--version             show program's version number and exit
--scan-time [SCAN_TIME]
                    Wait SCAN-TIME seconds between status checks.

I would prefer something like:

--scan-time [SCAN_TIME]
                    Wait SCAN-TIME seconds between status checks.
                    (Default = 5)

Peeking at the help formatter code revealed limited options. Is there a clever way to get argparse to print the default value for --scan-time in a similar fashion, or should I just subclass the help formatter?

Use the argparse.ArgumentDefaultsHelpFormatter formatter:

parser = argparse.ArgumentParser(
    # ... other options ...
    formatter_class=argparse.ArgumentDefaultsHelpFormatter)

To quote the documentation:

The other formatter class available, ArgumentDefaultsHelpFormatter, will add information about the default value of each of the arguments.

Note that this only applies to arguments that have help text defined; with no help value for an argument, there is no help message to add information about the default value to.

The exact output for your scan-time option then becomes:

  --scan-time [SCAN_TIME]
                        Wait SCAN-TIME seconds between status checks.
                        (default: 5)

Add '%(default)s' to the help parameter to control what is displayed.

parser.add_argument("--type", default="toto", choices=["toto","titi"],
                              help = "type (default: %(default)s)")

Wrapper class

This is the most reliable and DRY approach I’ve found so far to both show defaults and use another formatter such as argparse.RawTextHelpFormatter at the same time:

#!/usr/bin/env python3

import argparse

class ArgumentParserWithDefaults(argparse.ArgumentParser):
    def add_argument(self, *args, help=None, default=None, **kwargs):
        if help is not None:
            kwargs['help'] = help
        if default is not None and args[0] != '-h':
            kwargs['default'] = default
            if help is not None:
                kwargs['help'] += ' Default: {}'.format(default)
        super().add_argument(*args, **kwargs)

parser = ArgumentParserWithDefaults(
    formatter_class=argparse.RawTextHelpFormatter
)
parser.add_argument('-a', default=13, help='''my help
for a''')
parser.add_argument('-b', default=42, help='''my help
for b''')
parser.add_argument('--no-default', help='''my help
for no-default''')
parser.add_argument('--no-help', default=101)

parser.print_help()
print()
print(parser.parse_args())

Output:

usage: main.py [-h] [-a A] [-b B] [--no-default NO_DEFAULT]
               [--no-help NO_HELP]

optional arguments:
  -h, --help            show this help message and exit
  -a A                  my help
                        for a Default: 13
  -b B                  my help
                        for b Default: 42
  --no-default NO_DEFAULT
                        my help
                        for no-default
  --no-help NO_HELP

Namespace(a=13, b=42, no_default=None, no_help=101)

ArgumentDefaultsHelpFormatter + RawTextHelpFormatter multiple inheritance

Multiple inheritance just works, but it does not seem to be public API:

#!/usr/bin/env python3

import argparse

class RawTextArgumentDefaultsHelpFormatter(
        argparse.ArgumentDefaultsHelpFormatter,
        argparse.RawTextHelpFormatter
    ):
        pass

parser = argparse.ArgumentParser(
    formatter_class=RawTextArgumentDefaultsHelpFormatter
)
parser.add_argument('-a', default=13, help='''my help
for a''')
parser.add_argument('-b', default=42, help='''my help
for b''')
parser.print_help()

Output:

usage: a.py [-h] [-a A] [-b B]

optional arguments:
  -h, --help  show this help message and exit
  -a A        my help
              for a (default: 13)
  -b B        my help
              for b (default: 42)

It just works works because as we can see trivially from the sources https://github.com/python/cpython/blob/v3.6.5/Lib/argparse.py#L648 that:

• RawTextHelpFormatter implements _split_lines
• ArgumentDefaultsHelpFormatter implements _get_help_string

so we can guess that they will work together just fine.

However, this does not seem to be public API, and neither are the methods of formatter_class, so I don’t think there is a public API way to do it currently. argparse docstring says:

All other classes in this module are considered implementation details. (Also note that HelpFormatter and RawDescriptionHelpFormatter are only considered public as object names — the API of the formatter objects is still considered an implementation detail.)

See also: Customize argparse help message

Tested on Python 3.6.5.