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argparse.py
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2511 lines (2081 loc) · 93.5 KB
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# Author: Steven J. Bethard <steven.bethard@gmail.com>.
"""Command-line parsing library
This module is an optparse-inspired command-line parsing library that:
- handles both optional and positional arguments
- produces highly informative usage messages
- supports parsers that dispatch to sub-parsers
The following is a simple usage example that sums integers from the
command-line and writes the result to a file::
parser = argparse.ArgumentParser(
description='sum the integers at the command line')
parser.add_argument(
'integers', metavar='int', nargs='+', type=int,
help='an integer to be summed')
parser.add_argument(
'--log', default=sys.stdout, type=argparse.FileType('w'),
help='the file where the sum should be written')
args = parser.parse_args()
args.log.write('%s' % sum(args.integers))
args.log.close()
The module contains the following public classes:
- ArgumentParser -- The main entry point for command-line parsing. As the
example above shows, the add_argument() method is used to populate
the parser with actions for optional and positional arguments. Then
the parse_args() method is invoked to convert the args at the
command-line into an object with attributes.
- ArgumentError -- The exception raised by ArgumentParser objects when
there are errors with the parser's actions. Errors raised while
parsing the command-line are caught by ArgumentParser and emitted
as command-line messages.
- FileType -- A factory for defining types of files to be created. As the
example above shows, instances of FileType are typically passed as
the type= argument of add_argument() calls.
- Action -- The base class for parser actions. Typically actions are
selected by passing strings like 'store_true' or 'append_const' to
the action= argument of add_argument(). However, for greater
customization of ArgumentParser actions, subclasses of Action may
be defined and passed as the action= argument.
- HelpFormatter, RawDescriptionHelpFormatter, RawTextHelpFormatter,
ArgumentDefaultsHelpFormatter -- Formatter classes which
may be passed as the formatter_class= argument to the
ArgumentParser constructor. HelpFormatter is the default,
RawDescriptionHelpFormatter and RawTextHelpFormatter tell the parser
not to change the formatting for help text, and
ArgumentDefaultsHelpFormatter adds information about argument defaults
to the help.
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.)
"""
__version__ = '1.1'
__all__ = [
'ArgumentParser',
'ArgumentError',
'ArgumentTypeError',
'FileType',
'HelpFormatter',
'ArgumentDefaultsHelpFormatter',
'RawDescriptionHelpFormatter',
'RawTextHelpFormatter',
'MetavarTypeHelpFormatter',
'Namespace',
'Action',
'ONE_OR_MORE',
'OPTIONAL',
'PARSER',
'REMAINDER',
'SUPPRESS',
'ZERO_OR_MORE',
]
import os as _os
import re as _re
import sys as _sys
from gettext import gettext as _, ngettext
SUPPRESS = '==SUPPRESS=='
OPTIONAL = '?'
ZERO_OR_MORE = '*'
ONE_OR_MORE = '+'
PARSER = 'A...'
REMAINDER = '...'
_UNRECOGNIZED_ARGS_ATTR = '_unrecognized_args'
# =============================
# Utility functions and classes
# =============================
class _AttributeHolder(object):
"""Abstract base class that provides __repr__.
The __repr__ method returns a string in the format::
ClassName(attr=name, attr=name, ...)
The attributes are determined either by a class-level attribute,
'_kwarg_names', or by inspecting the instance __dict__.
"""
def __repr__(self):
type_name = type(self).__name__
arg_strings = []
star_args = {}
for arg in self._get_args():
arg_strings.append(repr(arg))
for name, value in self._get_kwargs():
if name.isidentifier():
arg_strings.append('%s=%r' % (name, value))
else:
star_args[name] = value
if star_args:
arg_strings.append('**%s' % repr(star_args))
return '%s(%s)' % (type_name, ', '.join(arg_strings))
def _get_kwargs(self):
return sorted(self.__dict__.items())
def _get_args(self):
return []
def _copy_items(items):
if items is None:
return []
# The copy module is used only in the 'append' and 'append_const'
# actions, and it is needed only when the default value isn't a list.
# Delay its import for speeding up the common case.
if type(items) is list:
return items[:]
import copy
return copy.copy(items)
# ===============
# Formatting Help
# ===============
class HelpFormatter(object):
"""Formatter for generating usage messages and argument help strings.
Only the name of this class is considered a public API. All the methods
provided by the class are considered an implementation detail.
"""
def __init__(self,
prog,
indent_increment=2,
max_help_position=24,
width=None):
# default setting for width
if width is None:
try:
width = int(_os.environ['COLUMNS'])
except (KeyError, ValueError):
width = 80
width -= 2
self._prog = prog
self._indent_increment = indent_increment
self._max_help_position = max_help_position
self._max_help_position = min(max_help_position,
max(width - 20, indent_increment * 2))
self._width = width
self._current_indent = 0
self._level = 0
self._action_max_length = 0
self._root_section = self._Section(self, None)
self._current_section = self._root_section
self._whitespace_matcher = _re.compile(r'\s+', _re.ASCII)
self._long_break_matcher = _re.compile(r'\n\n\n+')
# ===============================
# Section and indentation methods
# ===============================
def _indent(self):
self._current_indent += self._indent_increment
self._level += 1
def _dedent(self):
self._current_indent -= self._indent_increment
assert self._current_indent >= 0, 'Indent decreased below 0.'
self._level -= 1
class _Section(object):
def __init__(self, formatter, parent, heading=None):
self.formatter = formatter
self.parent = parent
self.heading = heading
self.items = []
def format_help(self):
# format the indented section
if self.parent is not None:
self.formatter._indent()
join = self.formatter._join_parts
item_help = join([func(*args) for func, args in self.items])
if self.parent is not None:
self.formatter._dedent()
# return nothing if the section was empty
if not item_help:
return ''
# add the heading if the section was non-empty
if self.heading is not SUPPRESS and self.heading is not None:
current_indent = self.formatter._current_indent
heading = '%*s%s:\n' % (current_indent, '', self.heading)
else:
heading = ''
# join the section-initial newline, the heading and the help
return join(['\n', heading, item_help, '\n'])
def _add_item(self, func, args):
self._current_section.items.append((func, args))
# ========================
# Message building methods
# ========================
def start_section(self, heading):
self._indent()
section = self._Section(self, self._current_section, heading)
self._add_item(section.format_help, [])
self._current_section = section
def end_section(self):
self._current_section = self._current_section.parent
self._dedent()
def add_text(self, text):
if text is not SUPPRESS and text is not None:
self._add_item(self._format_text, [text])
def add_usage(self, usage, actions, groups, prefix=None):
if usage is not SUPPRESS:
args = usage, actions, groups, prefix
self._add_item(self._format_usage, args)
def add_argument(self, action):
if action.help is not SUPPRESS:
# find all invocations
get_invocation = self._format_action_invocation
invocations = [get_invocation(action)]
for subaction in self._iter_indented_subactions(action):
invocations.append(get_invocation(subaction))
# update the maximum item length
invocation_length = max([len(s) for s in invocations])
action_length = invocation_length + self._current_indent
self._action_max_length = max(self._action_max_length,
action_length)
# add the item to the list
self._add_item(self._format_action, [action])
def add_arguments(self, actions):
for action in actions:
self.add_argument(action)
# =======================
# Help-formatting methods
# =======================
def format_help(self):
help = self._root_section.format_help()
if help:
help = self._long_break_matcher.sub('\n\n', help)
help = help.strip('\n') + '\n'
return help
def _join_parts(self, part_strings):
return ''.join([part
for part in part_strings
if part and part is not SUPPRESS])
def _format_usage(self, usage, actions, groups, prefix):
if prefix is None:
prefix = _('usage: ')
# if usage is specified, use that
if usage is not None:
usage = usage % dict(prog=self._prog)
# if no optionals or positionals are available, usage is just prog
elif usage is None and not actions:
usage = '%(prog)s' % dict(prog=self._prog)
# if optionals and positionals are available, calculate usage
elif usage is None:
prog = '%(prog)s' % dict(prog=self._prog)
# split optionals from positionals
optionals = []
positionals = []
for action in actions:
if action.option_strings:
optionals.append(action)
else:
positionals.append(action)
# build full usage string
format = self._format_actions_usage
action_usage = format(optionals + positionals, groups)
usage = ' '.join([s for s in [prog, action_usage] if s])
# wrap the usage parts if it's too long
text_width = self._width - self._current_indent
if len(prefix) + len(usage) > text_width:
# break usage into wrappable parts
part_regexp = (
r'(\(.*?\)+|\[.*?\]+|\S+)(?:\s|$)'
)
# FIXME: use regex below once we're compatible with sre
# part_regexp = (
# r'\(.*?\)+(?=\s|$)|'
# r'\[.*?\]+(?=\s|$)|'
# r'\S+'
# )
opt_usage = format(optionals, groups)
pos_usage = format(positionals, groups)
opt_parts = list(map(lambda t: t[0], _re.findall(part_regexp, opt_usage)))
pos_parts = list(map(lambda t: t[0], _re.findall(part_regexp, pos_usage)))
assert ' '.join(opt_parts) == opt_usage
assert ' '.join(pos_parts) == pos_usage
# helper for wrapping lines
def get_lines(parts, indent, prefix=None):
lines = []
line = []
if prefix is not None:
line_len = len(prefix) - 1
else:
line_len = len(indent) - 1
for part in parts:
if line_len + 1 + len(part) > text_width and line:
lines.append(indent + ' '.join(line))
line = []
line_len = len(indent) - 1
line.append(part)
line_len += len(part) + 1
if line:
lines.append(indent + ' '.join(line))
if prefix is not None:
lines[0] = lines[0][len(indent):]
return lines
# if prog is short, follow it with optionals or positionals
if len(prefix) + len(prog) <= 0.75 * text_width:
indent = ' ' * (len(prefix) + len(prog) + 1)
if opt_parts:
lines = get_lines([prog] + opt_parts, indent, prefix)
lines.extend(get_lines(pos_parts, indent))
elif pos_parts:
lines = get_lines([prog] + pos_parts, indent, prefix)
else:
lines = [prog]
# if prog is long, put it on its own line
else:
indent = ' ' * len(prefix)
parts = opt_parts + pos_parts
lines = get_lines(parts, indent)
if len(lines) > 1:
lines = []
lines.extend(get_lines(opt_parts, indent))
lines.extend(get_lines(pos_parts, indent))
lines = [prog] + lines
# join lines into usage
usage = '\n'.join(lines)
# prefix with 'usage:'
return '%s%s\n\n' % (prefix, usage)
def _format_actions_usage(self, actions, groups):
# find group indices and identify actions in groups
group_actions = set()
inserts = {}
for group in groups:
try:
start = actions.index(group._group_actions[0])
except ValueError:
continue
else:
end = start + len(group._group_actions)
if actions[start:end] == group._group_actions:
for action in group._group_actions:
group_actions.add(action)
if not group.required:
if start in inserts:
inserts[start] += ' ['
else:
inserts[start] = '['
inserts[end] = ']'
else:
if start in inserts:
inserts[start] += ' ('
else:
inserts[start] = '('
inserts[end] = ')'
for i in range(start + 1, end):
inserts[i] = '|'
# collect all actions format strings
parts = []
for i, action in enumerate(actions):
# suppressed arguments are marked with None
# remove | separators for suppressed arguments
if action.help is SUPPRESS:
parts.append(None)
if inserts.get(i) == '|':
inserts.pop(i)
elif inserts.get(i + 1) == '|':
inserts.pop(i + 1)
# produce all arg strings
elif not action.option_strings:
default = self._get_default_metavar_for_positional(action)
part = self._format_args(action, default)
# if it's in a group, strip the outer []
if action in group_actions:
if part[0] == '[' and part[-1] == ']':
part = part[1:-1]
# add the action string to the list
parts.append(part)
# produce the first way to invoke the option in brackets
else:
option_string = action.option_strings[0]
# if the Optional doesn't take a value, format is:
# -s or --long
if action.nargs == 0:
part = '%s' % option_string
# if the Optional takes a value, format is:
# -s ARGS or --long ARGS
else:
default = self._get_default_metavar_for_optional(action)
args_string = self._format_args(action, default)
part = '%s %s' % (option_string, args_string)
# make it look optional if it's not required or in a group
if not action.required and action not in group_actions:
part = '[%s]' % part
# add the action string to the list
parts.append(part)
# insert things at the necessary indices
for i in sorted(inserts, reverse=True):
parts[i:i] = [inserts[i]]
# join all the action items with spaces
text = ' '.join([item for item in parts if item is not None])
# clean up separators for mutually exclusive groups
open = r'[\[(]'
close = r'[\])]'
text = _re.sub(r'(%s) ' % open, r'\1', text)
text = _re.sub(r' (%s)' % close, r'\1', text)
text = _re.sub(r'%s *%s' % (open, close), r'', text)
text = _re.sub(r'\(([^|]*)\)', r'\1', text)
text = text.strip()
# return the text
return text
def _format_text(self, text):
if '%(prog)' in text:
text = text % dict(prog=self._prog)
text_width = max(self._width - self._current_indent, 11)
indent = ' ' * self._current_indent
return self._fill_text(text, text_width, indent) + '\n\n'
def _format_action(self, action):
# determine the required width and the entry label
help_position = min(self._action_max_length + 2,
self._max_help_position)
help_width = max(self._width - help_position, 11)
action_width = help_position - self._current_indent - 2
action_header = self._format_action_invocation(action)
# no help; start on same line and add a final newline
if not action.help:
tup = self._current_indent, '', action_header
action_header = '%*s%s\n' % tup
# short action name; start on the same line and pad two spaces
elif len(action_header) <= action_width:
tup = self._current_indent, '', action_width, action_header
action_header = '%*s%-*s ' % tup
indent_first = 0
# long action name; start on the next line
else:
tup = self._current_indent, '', action_header
action_header = '%*s%s\n' % tup
indent_first = help_position
# collect the pieces of the action help
parts = [action_header]
# if there was help for the action, add lines of help text
if action.help:
help_text = self._expand_help(action)
help_lines = self._split_lines(help_text, help_width)
parts.append('%*s%s\n' % (indent_first, '', help_lines[0]))
for line in help_lines[1:]:
parts.append('%*s%s\n' % (help_position, '', line))
# or add a newline if the description doesn't end with one
elif not action_header.endswith('\n'):
parts.append('\n')
# if there are any sub-actions, add their help as well
for subaction in self._iter_indented_subactions(action):
parts.append(self._format_action(subaction))
# return a single string
return self._join_parts(parts)
def _format_action_invocation(self, action):
if not action.option_strings:
default = self._get_default_metavar_for_positional(action)
metavar, = self._metavar_formatter(action, default)(1)
return metavar
else:
parts = []
# if the Optional doesn't take a value, format is:
# -s, --long
if action.nargs == 0:
parts.extend(action.option_strings)
# if the Optional takes a value, format is:
# -s ARGS, --long ARGS
else:
default = self._get_default_metavar_for_optional(action)
args_string = self._format_args(action, default)
for option_string in action.option_strings:
parts.append('%s %s' % (option_string, args_string))
return ', '.join(parts)
def _metavar_formatter(self, action, default_metavar):
if action.metavar is not None:
result = action.metavar
elif action.choices is not None:
choice_strs = [str(choice) for choice in action.choices]
result = '{%s}' % ','.join(choice_strs)
else:
result = default_metavar
def format(tuple_size):
if isinstance(result, tuple):
return result
else:
return (result, ) * tuple_size
return format
def _format_args(self, action, default_metavar):
get_metavar = self._metavar_formatter(action, default_metavar)
if action.nargs is None:
result = '%s' % get_metavar(1)
elif action.nargs == OPTIONAL:
result = '[%s]' % get_metavar(1)
elif action.nargs == ZERO_OR_MORE:
result = '[%s [%s ...]]' % get_metavar(2)
elif action.nargs == ONE_OR_MORE:
result = '%s [%s ...]' % get_metavar(2)
elif action.nargs == REMAINDER:
result = '...'
elif action.nargs == PARSER:
result = '%s ...' % get_metavar(1)
elif action.nargs == SUPPRESS:
result = ''
else:
formats = ['%s' for _ in range(action.nargs)]
result = ' '.join(formats) % get_metavar(action.nargs)
return result
def _expand_help(self, action):
params = dict(vars(action), prog=self._prog)
for name in list(params):
if params[name] is SUPPRESS:
del params[name]
for name in list(params):
if hasattr(params[name], '__name__'):
params[name] = params[name].__name__
if params.get('choices') is not None:
choices_str = ', '.join([str(c) for c in params['choices']])
params['choices'] = choices_str
return self._get_help_string(action) % params
def _iter_indented_subactions(self, action):
try:
get_subactions = action._get_subactions
except AttributeError:
pass
else:
self._indent()
yield from get_subactions()
self._dedent()
def _split_lines(self, text, width):
text = self._whitespace_matcher.sub(' ', text).strip()
# The textwrap module is used only for formatting help.
# Delay its import for speeding up the common usage of argparse.
import textwrap
return textwrap.wrap(text, width)
def _fill_text(self, text, width, indent):
text = self._whitespace_matcher.sub(' ', text).strip()
import textwrap
return textwrap.fill(text, width,
initial_indent=indent,
subsequent_indent=indent)
def _get_help_string(self, action):
return action.help
def _get_default_metavar_for_optional(self, action):
return action.dest.upper()
def _get_default_metavar_for_positional(self, action):
return action.dest
class RawDescriptionHelpFormatter(HelpFormatter):
"""Help message formatter which retains any formatting in descriptions.
Only the name of this class is considered a public API. All the methods
provided by the class are considered an implementation detail.
"""
def _fill_text(self, text, width, indent):
return ''.join(indent + line for line in text.splitlines(keepends=True))
class RawTextHelpFormatter(RawDescriptionHelpFormatter):
"""Help message formatter which retains formatting of all help text.
Only the name of this class is considered a public API. All the methods
provided by the class are considered an implementation detail.
"""
def _split_lines(self, text, width):
return text.splitlines()
class ArgumentDefaultsHelpFormatter(HelpFormatter):
"""Help message formatter which adds default values to argument help.
Only the name of this class is considered a public API. All the methods
provided by the class are considered an implementation detail.
"""
def _get_help_string(self, action):
help = action.help
if '%(default)' not in action.help:
if action.default is not SUPPRESS:
defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
if action.option_strings or action.nargs in defaulting_nargs:
help += ' (default: %(default)s)'
return help
class MetavarTypeHelpFormatter(HelpFormatter):
"""Help message formatter which uses the argument 'type' as the default
metavar value (instead of the argument 'dest')
Only the name of this class is considered a public API. All the methods
provided by the class are considered an implementation detail.
"""
def _get_default_metavar_for_optional(self, action):
return action.type.__name__
def _get_default_metavar_for_positional(self, action):
return action.type.__name__
# =====================
# Options and Arguments
# =====================
def _get_action_name(argument):
if argument is None:
return None
elif argument.option_strings:
return '/'.join(argument.option_strings)
elif argument.metavar not in (None, SUPPRESS):
return argument.metavar
elif argument.dest not in (None, SUPPRESS):
return argument.dest
else:
return None
class ArgumentError(Exception):
"""An error from creating or using an argument (optional or positional).
The string value of this exception is the message, augmented with
information about the argument that caused it.
"""
def __init__(self, argument, message):
self.argument_name = _get_action_name(argument)
self.message = message
def __str__(self):
if self.argument_name is None:
format = '%(message)s'
else:
format = 'argument %(argument_name)s: %(message)s'
return format % dict(message=self.message,
argument_name=self.argument_name)
class ArgumentTypeError(Exception):
"""An error from trying to convert a command line string to a type."""
pass
# ==============
# Action classes
# ==============
class Action(_AttributeHolder):
"""Information about how to convert command line strings to Python objects.
Action objects are used by an ArgumentParser to represent the information
needed to parse a single argument from one or more strings from the
command line. The keyword arguments to the Action constructor are also
all attributes of Action instances.
Keyword Arguments:
- option_strings -- A list of command-line option strings which
should be associated with this action.
- dest -- The name of the attribute to hold the created object(s)
- nargs -- The number of command-line arguments that should be
consumed. By default, one argument will be consumed and a single
value will be produced. Other values include:
- N (an integer) consumes N arguments (and produces a list)
- '?' consumes zero or one arguments
- '*' consumes zero or more arguments (and produces a list)
- '+' consumes one or more arguments (and produces a list)
Note that the difference between the default and nargs=1 is that
with the default, a single value will be produced, while with
nargs=1, a list containing a single value will be produced.
- const -- The value to be produced if the option is specified and the
option uses an action that takes no values.
- default -- The value to be produced if the option is not specified.
- type -- A callable that accepts a single string argument, and
returns the converted value. The standard Python types str, int,
float, and complex are useful examples of such callables. If None,
str is used.
- choices -- A container of values that should be allowed. If not None,
after a command-line argument has been converted to the appropriate
type, an exception will be raised if it is not a member of this
collection.
- required -- True if the action must always be specified at the
command line. This is only meaningful for optional command-line
arguments.
- help -- The help string describing the argument.
- metavar -- The name to be used for the option's argument with the
help string. If None, the 'dest' value will be used as the name.
"""
def __init__(self,
option_strings,
dest,
nargs=None,
const=None,
default=None,
type=None,
choices=None,
required=False,
help=None,
metavar=None):
self.option_strings = option_strings
self.dest = dest
self.nargs = nargs
self.const = const
self.default = default
self.type = type
self.choices = choices
self.required = required
self.help = help
self.metavar = metavar
def _get_kwargs(self):
names = [
'option_strings',
'dest',
'nargs',
'const',
'default',
'type',
'choices',
'help',
'metavar',
]
return [(name, getattr(self, name)) for name in names]
def __call__(self, parser, namespace, values, option_string=None):
raise NotImplementedError(_('.__call__() not defined'))
class _StoreAction(Action):
def __init__(self,
option_strings,
dest,
nargs=None,
const=None,
default=None,
type=None,
choices=None,
required=False,
help=None,
metavar=None):
if nargs == 0:
raise ValueError('nargs for store actions must be > 0; if you '
'have nothing to store, actions such as store '
'true or store const may be more appropriate')
if const is not None and nargs != OPTIONAL:
raise ValueError('nargs must be %r to supply const' % OPTIONAL)
super(_StoreAction, self).__init__(
option_strings=option_strings,
dest=dest,
nargs=nargs,
const=const,
default=default,
type=type,
choices=choices,
required=required,
help=help,
metavar=metavar)
def __call__(self, parser, namespace, values, option_string=None):
setattr(namespace, self.dest, values)
class _StoreConstAction(Action):
def __init__(self,
option_strings,
dest,
const,
default=None,
required=False,
help=None,
metavar=None):
super(_StoreConstAction, self).__init__(
option_strings=option_strings,
dest=dest,
nargs=0,
const=const,
default=default,
required=required,
help=help)
def __call__(self, parser, namespace, values, option_string=None):
setattr(namespace, self.dest, self.const)
class _StoreTrueAction(_StoreConstAction):
def __init__(self,
option_strings,
dest,
default=False,
required=False,
help=None):
super(_StoreTrueAction, self).__init__(
option_strings=option_strings,
dest=dest,
const=True,
default=default,
required=required,
help=help)
class _StoreFalseAction(_StoreConstAction):
def __init__(self,
option_strings,
dest,
default=True,
required=False,
help=None):
super(_StoreFalseAction, self).__init__(
option_strings=option_strings,
dest=dest,
const=False,
default=default,
required=required,
help=help)
class _AppendAction(Action):
def __init__(self,
option_strings,
dest,
nargs=None,
const=None,
default=None,
type=None,
choices=None,
required=False,
help=None,
metavar=None):
if nargs == 0:
raise ValueError('nargs for append actions must be > 0; if arg '
'strings are not supplying the value to append, '
'the append const action may be more appropriate')
if const is not None and nargs != OPTIONAL:
raise ValueError('nargs must be %r to supply const' % OPTIONAL)
super(_AppendAction, self).__init__(
option_strings=option_strings,
dest=dest,
nargs=nargs,
const=const,
default=default,
type=type,
choices=choices,
required=required,
help=help,
metavar=metavar)
def __call__(self, parser, namespace, values, option_string=None):
items = getattr(namespace, self.dest, None)
items = _copy_items(items)
items.append(values)
setattr(namespace, self.dest, items)
class _AppendConstAction(Action):
def __init__(self,
option_strings,
dest,
const,
default=None,
required=False,
help=None,
metavar=None):
super(_AppendConstAction, self).__init__(
option_strings=option_strings,
dest=dest,
nargs=0,
const=const,
default=default,
required=required,
help=help,
metavar=metavar)
def __call__(self, parser, namespace, values, option_string=None):
items = getattr(namespace, self.dest, None)
items = _copy_items(items)
items.append(self.const)
setattr(namespace, self.dest, items)