Lib/argparse.py (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/argparse.py ↗

This annotation covers advanced argparse features. See lib_argparse_detail (if it exists) for ArgumentParser, add_argument, and basic parsing.

Map

Lines	Symbol	Role
1-150	Action, built-in action types	_StoreAction, _StoreTrueAction, _AppendAction, etc.
151-350	FileType	Callable type that opens files
351-550	ArgumentGroup, MutuallyExclusiveGroup	Grouping and exclusion
551-800	_SubParsersAction	Subcommand support
801-1100	Type coercion and choices	type=int, choices=['a','b']
1101-1400	HelpFormatter, RawTextHelpFormatter	Help text formatting
1401-1800	metavar, dest derivation	Argument display names

Reading

FileType

# CPython: Lib/argparse.py:1215 FileType
class FileType:
    def __init__(self, mode='r', bufsize=-1, encoding=None, errors=None):
        self._mode = mode
        self._bufsize = bufsize
        ...

    def __call__(self, string):
        if string == '-':
            if 'r' in self._mode:
                return _sys.stdin
            elif 'w' in self._mode:
                return _sys.stdout
        try:
            return open(string, self._mode, self._bufsize, self._encoding, self._errors)
        except OSError as e:
            raise ArgumentTypeError(...)

type=FileType('r') opens the file and returns the file object as the argument value.

Subparsers

# CPython: Lib/argparse.py:1310 _SubParsersAction.__call__
def __call__(self, parser, namespace, values, option_string=None):
    parser_name = values[0]
    try:
        subparser = self._name_parser_map[parser_name]
    except KeyError:
        raise ArgumentError(self, 'invalid choice: %r (choose from %s)' % ...)
    # Parse remaining args with the subparser
    subnamespace = Namespace()
    subparser.parse_args(values[1:], subnamespace)
    for key, value in vars(subnamespace).items():
        setattr(namespace, key, value)

Custom Action

# CPython: Lib/argparse.py usage
class VerboseAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        count = getattr(namespace, self.dest, 0)
        setattr(namespace, self.dest, count + 1)

parser.add_argument('-v', action=VerboseAction, dest='verbosity', default=0)

Custom actions must define __call__(self, parser, namespace, values, option_string).

MutuallyExclusiveGroup

# CPython: Lib/argparse.py:1520 MutuallyExclusiveGroup._group_check_conflict
def _group_check_conflict(self, action):
    mutex_action = self._option_string_actions.get(option_string)
    if mutex_action and mutex_action.option_strings[0] in seen_non_default_actions:
        msg = _('not allowed with argument %s')
        raise ArgumentError(action, msg % mutex_action.option_strings[0])

gopy notes

argparse is pure Python and depends only on sys, os, re, textwrap, copy, and gettext. It is used by many CLI tools. In gopy the module can be run from the stdlib once textwrap.fill and re.match work correctly. The key dependency is sys.argv being correctly populated.