16.4. "argparse" --- 命令行选项、参数和子命令解析器
***************************************************

3.2 新版功能.

**源代码：** Lib/argparse.py

======================================================================


入门教程
^^^^^^^^

此页面包含 API 参考信息。有关 Python 命令行解析的更文雅的介绍，请参阅
<argparse-tutorial>。

"argparse" 模块可以轻松编写用户友好的命令行接口。程序定义它需要的参数
，然后 "argparse" 将弄清如何从 "sys.argv" 解析出那些参数。 "argparse"
模块还会自动生成帮助和使用手册，并在用户给程序传入无效参数时报出错误信
息。


16.4.1. 示例
============

以下代码是一个 Python 程序，它获取一个整数列表并计算合计或者最大值：

   import argparse

   parser = argparse.ArgumentParser(description='Process some integers.')
   parser.add_argument('integers', metavar='N', type=int, nargs='+',
                       help='an integer for the accumulator')
   parser.add_argument('--sum', dest='accumulate', action='store_const',
                       const=sum, default=max,
                       help='sum the integers (default: find the max)')

   args = parser.parse_args()
   print(args.accumulate(args.integers))

假设上面的 Python 代码保存在名为 "prog.py" 的文件中，它可以在命令行运
行并提供有用的帮助消息：

   $ python prog.py -h
   usage: prog.py [-h] [--sum] N [N ...]

   Process some integers.

   positional arguments:
    N           an integer for the accumulator

   optional arguments:
    -h, --help  show this help message and exit
    --sum       sum the integers (default: find the max)

使用适当的参数运行时，它会输出命令行传入整数的总和或者最大值：

   $ python prog.py 1 2 3 4
   4

   $ python prog.py 1 2 3 4 --sum
   10

如果传入无效参数，则会报出错误：

   $ python prog.py a b c
   usage: prog.py [-h] [--sum] N [N ...]
   prog.py: error: argument N: invalid int value: 'a'

以下部分将引导你完成这个示例。


16.4.1.1. 创建一个解析器
------------------------

使用 "argparse" 的第一步是创建一个 "ArgumentParser" 对象：

   >>> parser = argparse.ArgumentParser(description='Process some integers.')

"ArgumentParser" 对象包含将命令行解析成 Python 数据类型所需的全部信息
。


16.4.1.2. 添加参数
------------------

给一个 "ArgumentParser" 添加程序参数信息是通过调用 "add_argument()" 方
法完成的。通常，这些调用指定 "ArgumentParser" 如何获取命令行字符串并将
其转换为对象。这些信息在 "parse_args()" 调用时被存储和使用。例如：

   >>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
   ...                     help='an integer for the accumulator')
   >>> parser.add_argument('--sum', dest='accumulate', action='store_const',
   ...                     const=sum, default=max,
   ...                     help='sum the integers (default: find the max)')

稍后，调用 "parse_args()" 将返回一个具有 "integers" 和 "accumulate" 两
个属性的对象。"integers" 属性将是一个包含一个或多个整数的列表，而
"accumulate" 属性当命令行中指定了 "--sum" 参数时将是 "sum()" 函数，否
则则是 "max()" 函数。


16.4.1.3. 解析参数
------------------

"ArgumentParser" 通过 "parse_args()" 方法解析参数。它将检查命令行，把
每个参数转换为适当的类型然后调用相应的操作。在大多数情况下，这意味着一
个简单的 "Namespace" 对象将从命令行参数中解析出的属性构建：

   >>> parser.parse_args(['--sum', '7', '-1', '42'])
   Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])

在脚本中，通常 "parse_args()" 会被不带参数调用，而 "ArgumentParser" 将
自动从 "sys.argv" 中确定命令行参数。


16.4.2. ArgumentParser 对象
===========================

class argparse.ArgumentParser(prog=None, usage=None, description=None, epilog=None, parents=[], formatter_class=argparse.HelpFormatter, prefix_chars='-', fromfile_prefix_chars=None, argument_default=None, conflict_handler='error', add_help=True, allow_abbrev=True)

   创建一个新的 "ArgumentParser" 对象。所有的参数都应当作为关键字参数
   传入。每个参数在下面都有它更详细的描述，但简而言之，它们是：

   * prog - 程序的名称（默认："sys.argv[0]"）

   * usage - 描述程序用途的字符串（默认值：从添加到解析器的参数生成
     ）

   * description - 在参数帮助文档之前显示的文本（默认值：无）

   * epilog - 在参数帮助文档之后显示的文本（默认值：无）

   * parents - 一个 "ArgumentParser" 对象的列表，它们的参数也应包含
     在 内

   * formatter_class - 用于自定义帮助文档输出格式的类

   * prefix_chars - 可选参数的前缀字符集合（默认值：'-'）

   * fromfile_prefix_chars - 当需要从文件中读取其他参数时，用于标识
     文 件名的前缀字符集合（默认值："None"）

   * argument_default - 参数的全局默认值（默认值： "None"）

   * conflict_handler - 解决冲突选项的策略（通常是不必要的）

   * add_help - 为解析器添加一个 "-h/--help" 选项（默认值： "True"）

   * allow_abbrev - 如果缩写是无歧义的，则允许缩写长选项 （默认值：
     "True"）

   在 3.5 版更改: 添加 *allow_abbrev* 参数。

以下部分描述这些参数如何使用。


16.4.2.1. prog
--------------

默认情况下，"ArgumentParser" 对象使用 "sys.argv[0]" 来确定如何在帮助消
息中显示程序名称。这一默认值几乎总是可取的，因为它将使帮助消息与从命令
行调用此程序的方式相匹配。例如，对于有如下代码的名为 "myprogram.py" 的
文件：

   import argparse
   parser = argparse.ArgumentParser()
   parser.add_argument('--foo', help='foo help')
   args = parser.parse_args()

该程序的帮助信息将显示 "myprogram.py" 作为程序名称（无论程序从何处被调
用）：

   $ python myprogram.py --help
   usage: myprogram.py [-h] [--foo FOO]

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO   foo help
   $ cd ..
   $ python subdir/myprogram.py --help
   usage: myprogram.py [-h] [--foo FOO]

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO   foo help

要更改这样的默认行为，可以使用 "prog=" 参数为 "ArgumentParser" 提供另
一个值：

   >>> parser = argparse.ArgumentParser(prog='myprogram')
   >>> parser.print_help()
   usage: myprogram [-h]

   optional arguments:
    -h, --help  show this help message and exit

需要注意的是，无论是从 "sys.argv[0]" 或是从 "prog=" 参数确定的程序名称
，都可以在帮助消息里通过 "%(prog)s" 格式串来引用。

   >>> parser = argparse.ArgumentParser(prog='myprogram')
   >>> parser.add_argument('--foo', help='foo of the %(prog)s program')
   >>> parser.print_help()
   usage: myprogram [-h] [--foo FOO]

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO   foo of the myprogram program


16.4.2.2. usage
---------------

默认情况下，"ArgumentParser" 根据它包含的参数来构建用法消息：

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('--foo', nargs='?', help='foo help')
   >>> parser.add_argument('bar', nargs='+', help='bar help')
   >>> parser.print_help()
   usage: PROG [-h] [--foo [FOO]] bar [bar ...]

   positional arguments:
    bar          bar help

   optional arguments:
    -h, --help   show this help message and exit
    --foo [FOO]  foo help

可以通过 "usage=" 关键字参数覆盖这一默认消息：

   >>> parser = argparse.ArgumentParser(prog='PROG', usage='%(prog)s [options]')
   >>> parser.add_argument('--foo', nargs='?', help='foo help')
   >>> parser.add_argument('bar', nargs='+', help='bar help')
   >>> parser.print_help()
   usage: PROG [options]

   positional arguments:
    bar          bar help

   optional arguments:
    -h, --help   show this help message and exit
    --foo [FOO]  foo help

在用法消息中可以使用 "%(prog)s" 格式说明符来填入程序名称。


16.4.2.3. description
---------------------

大多数对 "ArgumentParser" 构造方法的调用都会使用 "description=" 关键字
参数。这个参数简要描述这个程度做什么以及怎么做。在帮助消息中，这个描述
会显示在命令行用法字符串和各种参数的帮助消息之间：

   >>> parser = argparse.ArgumentParser(description='A foo that bars')
   >>> parser.print_help()
   usage: argparse.py [-h]

   A foo that bars

   optional arguments:
    -h, --help  show this help message and exit

By default, the description will be line-wrapped so that it fits
within the given space.  To change this behavior, see the
formatter_class argument.


16.4.2.4. epilog
----------------

Some programs like to display additional description of the program
after the description of the arguments.  Such text can be specified
using the "epilog=" argument to "ArgumentParser":

   >>> parser = argparse.ArgumentParser(
   ...     description='A foo that bars',
   ...     epilog="And that's how you'd foo a bar")
   >>> parser.print_help()
   usage: argparse.py [-h]

   A foo that bars

   optional arguments:
    -h, --help  show this help message and exit

   And that's how you'd foo a bar

As with the description argument, the "epilog=" text is by default
line-wrapped, but this behavior can be adjusted with the
formatter_class argument to "ArgumentParser".


16.4.2.5. parents
-----------------

Sometimes, several parsers share a common set of arguments. Rather
than repeating the definitions of these arguments, a single parser
with all the shared arguments and passed to "parents=" argument to
"ArgumentParser" can be used.  The "parents=" argument takes a list of
"ArgumentParser" objects, collects all the positional and optional
actions from them, and adds these actions to the "ArgumentParser"
object being constructed:

   >>> parent_parser = argparse.ArgumentParser(add_help=False)
   >>> parent_parser.add_argument('--parent', type=int)

   >>> foo_parser = argparse.ArgumentParser(parents=[parent_parser])
   >>> foo_parser.add_argument('foo')
   >>> foo_parser.parse_args(['--parent', '2', 'XXX'])
   Namespace(foo='XXX', parent=2)

   >>> bar_parser = argparse.ArgumentParser(parents=[parent_parser])
   >>> bar_parser.add_argument('--bar')
   >>> bar_parser.parse_args(['--bar', 'YYY'])
   Namespace(bar='YYY', parent=None)

Note that most parent parsers will specify "add_help=False".
Otherwise, the "ArgumentParser" will see two "-h/--help" options (one
in the parent and one in the child) and raise an error.

注解: You must fully initialize the parsers before passing them via
  "parents=". If you change the parent parsers after the child parser,
  those changes will not be reflected in the child.


16.4.2.6. formatter_class
-------------------------

"ArgumentParser" objects allow the help formatting to be customized by
specifying an alternate formatting class.  Currently, there are four
such classes:

class argparse.RawDescriptionHelpFormatter
class argparse.RawTextHelpFormatter
class argparse.ArgumentDefaultsHelpFormatter
class argparse.MetavarTypeHelpFormatter

"RawDescriptionHelpFormatter" and "RawTextHelpFormatter" give more
control over how textual descriptions are displayed. By default,
"ArgumentParser" objects line-wrap the description and epilog texts in
command-line help messages:

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     description='''this description
   ...         was indented weird
   ...             but that is okay''',
   ...     epilog='''
   ...             likewise for this epilog whose whitespace will
   ...         be cleaned up and whose words will be wrapped
   ...         across a couple lines''')
   >>> parser.print_help()
   usage: PROG [-h]

   this description was indented weird but that is okay

   optional arguments:
    -h, --help  show this help message and exit

   likewise for this epilog whose whitespace will be cleaned up and whose words
   will be wrapped across a couple lines

Passing "RawDescriptionHelpFormatter" as "formatter_class=" indicates
that description and epilog are already correctly formatted and should
not be line-wrapped:

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     formatter_class=argparse.RawDescriptionHelpFormatter,
   ...     description=textwrap.dedent('''\
   ...         Please do not mess up this text!
   ...         --------------------------------
   ...             I have indented it
   ...             exactly the way
   ...             I want it
   ...         '''))
   >>> parser.print_help()
   usage: PROG [-h]

   Please do not mess up this text!
   --------------------------------
      I have indented it
      exactly the way
      I want it

   optional arguments:
    -h, --help  show this help message and exit

"RawTextHelpFormatter" maintains whitespace for all sorts of help
text, including argument descriptions. However, multiple new lines are
replaced with one. If you wish to preserve multiple blank lines, add
spaces between the newlines.

"ArgumentDefaultsHelpFormatter" automatically adds information about
default values to each of the argument help messages:

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     formatter_class=argparse.ArgumentDefaultsHelpFormatter)
   >>> parser.add_argument('--foo', type=int, default=42, help='FOO!')
   >>> parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!')
   >>> parser.print_help()
   usage: PROG [-h] [--foo FOO] [bar [bar ...]]

   positional arguments:
    bar         BAR! (default: [1, 2, 3])

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO   FOO! (default: 42)

"MetavarTypeHelpFormatter" uses the name of the type argument for each
argument as the display name for its values (rather than using the
dest as the regular formatter does):

   >>> parser = argparse.ArgumentParser(
   ...     prog='PROG',
   ...     formatter_class=argparse.MetavarTypeHelpFormatter)
   >>> parser.add_argument('--foo', type=int)
   >>> parser.add_argument('bar', type=float)
   >>> parser.print_help()
   usage: PROG [-h] [--foo int] float

   positional arguments:
     float

   optional arguments:
     -h, --help  show this help message and exit
     --foo int


16.4.2.7. prefix_chars
----------------------

Most command-line options will use "-" as the prefix, e.g. "-f/--foo".
Parsers that need to support different or additional prefix
characters, e.g. for options like "+f" or "/foo", may specify them
using the "prefix_chars=" argument to the ArgumentParser constructor:

   >>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='-+')
   >>> parser.add_argument('+f')
   >>> parser.add_argument('++bar')
   >>> parser.parse_args('+f X ++bar Y'.split())
   Namespace(bar='Y', f='X')

The "prefix_chars=" argument defaults to "'-'". Supplying a set of
characters that does not include "-" will cause "-f/--foo" options to
be disallowed.


16.4.2.8. fromfile_prefix_chars
-------------------------------

Sometimes, for example when dealing with a particularly long argument
lists, it may make sense to keep the list of arguments in a file
rather than typing it out at the command line.  If the
"fromfile_prefix_chars=" argument is given to the "ArgumentParser"
constructor, then arguments that start with any of the specified
characters will be treated as files, and will be replaced by the
arguments they contain.  For example:

   >>> with open('args.txt', 'w') as fp:
   ...     fp.write('-f\nbar')
   >>> parser = argparse.ArgumentParser(fromfile_prefix_chars='@')
   >>> parser.add_argument('-f')
   >>> parser.parse_args(['-f', 'foo', '@args.txt'])
   Namespace(f='bar')

Arguments read from a file must by default be one per line (but see
also "convert_arg_line_to_args()") and are treated as if they were in
the same place as the original file referencing argument on the
command line.  So in the example above, the expression "['-f', 'foo',
'@args.txt']" is considered equivalent to the expression "['-f',
'foo', '-f', 'bar']".

The "fromfile_prefix_chars=" argument defaults to "None", meaning that
arguments will never be treated as file references.


16.4.2.9. argument_default
--------------------------

Generally, argument defaults are specified either by passing a default
to "add_argument()" or by calling the "set_defaults()" methods with a
specific set of name-value pairs.  Sometimes however, it may be useful
to specify a single parser-wide default for arguments.  This can be
accomplished by passing the "argument_default=" keyword argument to
"ArgumentParser".  For example, to globally suppress attribute
creation on "parse_args()" calls, we supply
"argument_default=SUPPRESS":

   >>> parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
   >>> parser.add_argument('--foo')
   >>> parser.add_argument('bar', nargs='?')
   >>> parser.parse_args(['--foo', '1', 'BAR'])
   Namespace(bar='BAR', foo='1')
   >>> parser.parse_args([])
   Namespace()


16.4.2.10. allow_abbrev
-----------------------

Normally, when you pass an argument list to the "parse_args()" method
of an "ArgumentParser", it recognizes abbreviations of long options.

This feature can be disabled by setting "allow_abbrev" to "False":

   >>> parser = argparse.ArgumentParser(prog='PROG', allow_abbrev=False)
   >>> parser.add_argument('--foobar', action='store_true')
   >>> parser.add_argument('--foonley', action='store_false')
   >>> parser.parse_args(['--foon'])
   usage: PROG [-h] [--foobar] [--foonley]
   PROG: error: unrecognized arguments: --foon

3.5 新版功能.


16.4.2.11. conflict_handler
---------------------------

"ArgumentParser" objects do not allow two actions with the same option
string.  By default, "ArgumentParser" objects raise an exception if an
attempt is made to create an argument with an option string that is
already in use:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-f', '--foo', help='old foo help')
   >>> parser.add_argument('--foo', help='new foo help')
   Traceback (most recent call last):
    ..
   ArgumentError: argument --foo: conflicting option string(s): --foo

Sometimes (e.g. when using parents) it may be useful to simply
override any older arguments with the same option string.  To get this
behavior, the value "'resolve'" can be supplied to the
"conflict_handler=" argument of "ArgumentParser":

   >>> parser = argparse.ArgumentParser(prog='PROG', conflict_handler='resolve')
   >>> parser.add_argument('-f', '--foo', help='old foo help')
   >>> parser.add_argument('--foo', help='new foo help')
   >>> parser.print_help()
   usage: PROG [-h] [-f FOO] [--foo FOO]

   optional arguments:
    -h, --help  show this help message and exit
    -f FOO      old foo help
    --foo FOO   new foo help

Note that "ArgumentParser" objects only remove an action if all of its
option strings are overridden.  So, in the example above, the old
"-f/--foo" action is retained as the "-f" action, because only the "--
foo" option string was overridden.


16.4.2.12. add_help
-------------------

By default, ArgumentParser objects add an option which simply displays
the parser's help message. For example, consider a file named
"myprogram.py" containing the following code:

   import argparse
   parser = argparse.ArgumentParser()
   parser.add_argument('--foo', help='foo help')
   args = parser.parse_args()

If "-h" or "--help" is supplied at the command line, the
ArgumentParser help will be printed:

   $ python myprogram.py --help
   usage: myprogram.py [-h] [--foo FOO]

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO   foo help

Occasionally, it may be useful to disable the addition of this help
option. This can be achieved by passing "False" as the "add_help="
argument to "ArgumentParser":

   >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
   >>> parser.add_argument('--foo', help='foo help')
   >>> parser.print_help()
   usage: PROG [--foo FOO]

   optional arguments:
    --foo FOO  foo help

The help option is typically "-h/--help". The exception to this is if
the "prefix_chars=" is specified and does not include "-", in which
case "-h" and "--help" are not valid options.  In this case, the first
character in "prefix_chars" is used to prefix the help options:

   >>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='+/')
   >>> parser.print_help()
   usage: PROG [+h]

   optional arguments:
     +h, ++help  show this help message and exit


16.4.3. The add_argument() method
=================================

ArgumentParser.add_argument(name or flags...[, action][, nargs][, const][, default][, type][, choices][, required][, help][, metavar][, dest])

   Define how a single command-line argument should be parsed.  Each
   parameter has its own more detailed description below, but in short
   they are:

   * name or flags - Either a name or a list of option strings, e.g.
     "foo" or "-f, --foo".

   * action - The basic type of action to be taken when this
     argument is encountered at the command line.

   * nargs - The number of command-line arguments that should be
     consumed.

   * const - A constant value required by some action and nargs
     selections.

   * default - The value produced if the argument is absent from the
     command line.

   * type - The type to which the command-line argument should be
     converted.

   * choices - A container of the allowable values for the argument.

   * required - Whether or not the command-line option may be
     omitted (optionals only).

   * help - A brief description of what the argument does.

   * metavar - A name for the argument in usage messages.

   * dest - The name of the attribute to be added to the object
     returned by "parse_args()".

以下部分描述这些参数如何使用。


16.4.3.1. name or flags
-----------------------

The "add_argument()" method must know whether an optional argument,
like "-f" or "--foo", or a positional argument, like a list of
filenames, is expected.  The first arguments passed to
"add_argument()" must therefore be either a series of flags, or a
simple argument name.  For example, an optional argument could be
created like:

   >>> parser.add_argument('-f', '--foo')

while a positional argument could be created like:

   >>> parser.add_argument('bar')

When "parse_args()" is called, optional arguments will be identified
by the "-" prefix, and the remaining arguments will be assumed to be
positional:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-f', '--foo')
   >>> parser.add_argument('bar')
   >>> parser.parse_args(['BAR'])
   Namespace(bar='BAR', foo=None)
   >>> parser.parse_args(['BAR', '--foo', 'FOO'])
   Namespace(bar='BAR', foo='FOO')
   >>> parser.parse_args(['--foo', 'FOO'])
   usage: PROG [-h] [-f FOO] bar
   PROG: error: the following arguments are required: bar


16.4.3.2. action
----------------

"ArgumentParser" objects associate command-line arguments with
actions.  These actions can do just about anything with the command-
line arguments associated with them, though most actions simply add an
attribute to the object returned by "parse_args()".  The "action"
keyword argument specifies how the command-line arguments should be
handled. The supplied actions are:

* "'store'" - This just stores the argument's value.  This is the
  default action. For example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo')
     >>> parser.parse_args('--foo 1'.split())
     Namespace(foo='1')

* "'store_const'" - This stores the value specified by the const
  keyword argument.  The "'store_const'" action is most commonly used
  with optional arguments that specify some sort of flag.  For
  example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', action='store_const', const=42)
     >>> parser.parse_args(['--foo'])
     Namespace(foo=42)

* "'store_true'" and "'store_false'" - These are special cases of
  "'store_const'" used for storing the values "True" and "False"
  respectively.  In addition, they create default values of "False"
  and "True" respectively.  For example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', action='store_true')
     >>> parser.add_argument('--bar', action='store_false')
     >>> parser.add_argument('--baz', action='store_false')
     >>> parser.parse_args('--foo --bar'.split())
     Namespace(foo=True, bar=False, baz=True)

* "'append'" - This stores a list, and appends each argument value
  to the list.  This is useful to allow an option to be specified
  multiple times. Example usage:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', action='append')
     >>> parser.parse_args('--foo 1 --foo 2'.split())
     Namespace(foo=['1', '2'])

* "'append_const'" - This stores a list, and appends the value
  specified by the const keyword argument to the list.  (Note that the
  const keyword argument defaults to "None".)  The "'append_const'"
  action is typically useful when multiple arguments need to store
  constants to the same list. For example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--str', dest='types', action='append_const', const=str)
     >>> parser.add_argument('--int', dest='types', action='append_const', const=int)
     >>> parser.parse_args('--str --int'.split())
     Namespace(types=[<class 'str'>, <class 'int'>])

* "'count'" - This counts the number of times a keyword argument
  occurs. For example, this is useful for increasing verbosity levels:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--verbose', '-v', action='count')
     >>> parser.parse_args(['-vvv'])
     Namespace(verbose=3)

* "'help'" - This prints a complete help message for all the options
  in the current parser and then exits. By default a help action is
  automatically added to the parser. See "ArgumentParser" for details
  of how the output is created.

* "'version'" - This expects a "version=" keyword argument in the
  "add_argument()" call, and prints version information and exits when
  invoked:

     >>> import argparse
     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> parser.add_argument('--version', action='version', version='%(prog)s 2.0')
     >>> parser.parse_args(['--version'])
     PROG 2.0

You may also specify an arbitrary action by passing an Action subclass
or other object that implements the same interface.  The recommended
way to do this is to extend "Action", overriding the "__call__" method
and optionally the "__init__" method.

An example of a custom action:

   >>> class FooAction(argparse.Action):
   ...     def __init__(self, option_strings, dest, nargs=None, **kwargs):
   ...         if nargs is not None:
   ...             raise ValueError("nargs not allowed")
   ...         super(FooAction, self).__init__(option_strings, dest, **kwargs)
   ...     def __call__(self, parser, namespace, values, option_string=None):
   ...         print('%r %r %r' % (namespace, values, option_string))
   ...         setattr(namespace, self.dest, values)
   ...
   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', action=FooAction)
   >>> parser.add_argument('bar', action=FooAction)
   >>> args = parser.parse_args('1 --foo 2'.split())
   Namespace(bar=None, foo=None) '1' None
   Namespace(bar='1', foo=None) '2' '--foo'
   >>> args
   Namespace(bar='1', foo='2')

For more details, see "Action".


16.4.3.3. nargs
---------------

ArgumentParser objects usually associate a single command-line
argument with a single action to be taken.  The "nargs" keyword
argument associates a different number of command-line arguments with
a single action.  The supported values are:

* "N" (an integer).  "N" arguments from the command line will be
  gathered together into a list.  For example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', nargs=2)
     >>> parser.add_argument('bar', nargs=1)
     >>> parser.parse_args('c --foo a b'.split())
     Namespace(bar=['c'], foo=['a', 'b'])

  Note that "nargs=1" produces a list of one item.  This is different
  from the default, in which the item is produced by itself.

* "'?'". One argument will be consumed from the command line if
  possible, and produced as a single item.  If no command-line
  argument is present, the value from default will be produced.  Note
  that for optional arguments, there is an additional case - the
  option string is present but not followed by a command-line
  argument.  In this case the value from const will be produced.  Some
  examples to illustrate this:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', nargs='?', const='c', default='d')
     >>> parser.add_argument('bar', nargs='?', default='d')
     >>> parser.parse_args(['XX', '--foo', 'YY'])
     Namespace(bar='XX', foo='YY')
     >>> parser.parse_args(['XX', '--foo'])
     Namespace(bar='XX', foo='c')
     >>> parser.parse_args([])
     Namespace(bar='d', foo='d')

  One of the more common uses of "nargs='?'" is to allow optional
  input and output files:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'),
     ...                     default=sys.stdin)
     >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'),
     ...                     default=sys.stdout)
     >>> parser.parse_args(['input.txt', 'output.txt'])
     Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>,
               outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>)
     >>> parser.parse_args([])
     Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>,
               outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)

* "'*'".  All command-line arguments present are gathered into a
  list. Note that it generally doesn't make much sense to have more
  than one positional argument with "nargs='*'", but multiple optional
  arguments with "nargs='*'" is possible.  For example:

     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', nargs='*')
     >>> parser.add_argument('--bar', nargs='*')
     >>> parser.add_argument('baz', nargs='*')
     >>> parser.parse_args('a b --foo x y --bar 1 2'.split())
     Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])

* "'+'". Just like "'*'", all command-line args present are gathered
  into a list.  Additionally, an error message will be generated if
  there wasn't at least one command-line argument present.  For
  example:

     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> parser.add_argument('foo', nargs='+')
     >>> parser.parse_args(['a', 'b'])
     Namespace(foo=['a', 'b'])
     >>> parser.parse_args([])
     usage: PROG [-h] foo [foo ...]
     PROG: error: the following arguments are required: foo

* "argparse.REMAINDER".  All the remaining command-line arguments
  are gathered into a list.  This is commonly useful for command line
  utilities that dispatch to other command line utilities:

     >>> parser = argparse.ArgumentParser(prog='PROG')
     >>> parser.add_argument('--foo')
     >>> parser.add_argument('command')
     >>> parser.add_argument('args', nargs=argparse.REMAINDER)
     >>> print(parser.parse_args('--foo B cmd --arg1 XX ZZ'.split()))
     Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')

If the "nargs" keyword argument is not provided, the number of
arguments consumed is determined by the action.  Generally this means
a single command-line argument will be consumed and a single item (not
a list) will be produced.


16.4.3.4. const
---------------

The "const" argument of "add_argument()" is used to hold constant
values that are not read from the command line but are required for
the various "ArgumentParser" actions.  The two most common uses of it
are:

* When "add_argument()" is called with "action='store_const'" or
  "action='append_const'".  These actions add the "const" value to one
  of the attributes of the object returned by "parse_args()". See the
  action description for examples.

* When "add_argument()" is called with option strings (like "-f" or
  " --foo") and "nargs='?'".  This creates an optional argument that
  can be followed by zero or one command-line arguments. When parsing
  the command line, if the option string is encountered with no
  command- line argument following it, the value of "const" will be
  assumed instead. See the nargs description for examples.

With the "'store_const'" and "'append_const'" actions, the "const"
keyword argument must be given.  For other actions, it defaults to
"None".


16.4.3.5. default
-----------------

All optional arguments and some positional arguments may be omitted at
the command line.  The "default" keyword argument of "add_argument()",
whose value defaults to "None", specifies what value should be used if
the command-line argument is not present. For optional arguments, the
"default" value is used when the option string was not present at the
command line:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', default=42)
   >>> parser.parse_args(['--foo', '2'])
   Namespace(foo='2')
   >>> parser.parse_args([])
   Namespace(foo=42)

If the "default" value is a string, the parser parses the value as if
it were a command-line argument.  In particular, the parser applies
any type conversion argument, if provided, before setting the
attribute on the "Namespace" return value.  Otherwise, the parser uses
the value as is:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--length', default='10', type=int)
   >>> parser.add_argument('--width', default=10.5, type=int)
   >>> parser.parse_args()
   Namespace(length=10, width=10.5)

For positional arguments with nargs equal to "?" or "*", the "default"
value is used when no command-line argument was present:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('foo', nargs='?', default=42)
   >>> parser.parse_args(['a'])
   Namespace(foo='a')
   >>> parser.parse_args([])
   Namespace(foo=42)

Providing "default=argparse.SUPPRESS" causes no attribute to be added
if the command-line argument was not present:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', default=argparse.SUPPRESS)
   >>> parser.parse_args([])
   Namespace()
   >>> parser.parse_args(['--foo', '1'])
   Namespace(foo='1')


16.4.3.6. 类型
--------------

By default, "ArgumentParser" objects read command-line arguments in as
simple strings. However, quite often the command-line string should
instead be interpreted as another type, like a "float" or "int".  The
"type" keyword argument of "add_argument()" allows any necessary type-
checking and type conversions to be performed.  Common built-in types
and functions can be used directly as the value of the "type"
argument:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('foo', type=int)
   >>> parser.add_argument('bar', type=open)
   >>> parser.parse_args('2 temp.txt'.split())
   Namespace(bar=<_io.TextIOWrapper name='temp.txt' encoding='UTF-8'>, foo=2)

See the section on the default keyword argument for information on
when the "type" argument is applied to default arguments.

To ease the use of various types of files, the argparse module
provides the factory FileType which takes the "mode=", "bufsize=",
"encoding=" and "errors=" arguments of the "open()" function.  For
example, "FileType('w')" can be used to create a writable file:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('bar', type=argparse.FileType('w'))
   >>> parser.parse_args(['out.txt'])
   Namespace(bar=<_io.TextIOWrapper name='out.txt' encoding='UTF-8'>)

"type=" can take any callable that takes a single string argument and
returns the converted value:

   >>> def perfect_square(string):
   ...     value = int(string)
   ...     sqrt = math.sqrt(value)
   ...     if sqrt != int(sqrt):
   ...         msg = "%r is not a perfect square" % string
   ...         raise argparse.ArgumentTypeError(msg)
   ...     return value
   ...
   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=perfect_square)
   >>> parser.parse_args(['9'])
   Namespace(foo=9)
   >>> parser.parse_args(['7'])
   usage: PROG [-h] foo
   PROG: error: argument foo: '7' is not a perfect square

The choices keyword argument may be more convenient for type checkers
that simply check against a range of values:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('foo', type=int, choices=range(5, 10))
   >>> parser.parse_args(['7'])
   Namespace(foo=7)
   >>> parser.parse_args(['11'])
   usage: PROG [-h] {5,6,7,8,9}
   PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)

See the choices section for more details.


16.4.3.7. choices
-----------------

Some command-line arguments should be selected from a restricted set
of values. These can be handled by passing a container object as the
*choices* keyword argument to "add_argument()".  When the command line
is parsed, argument values will be checked, and an error message will
be displayed if the argument was not one of the acceptable values:

   >>> parser = argparse.ArgumentParser(prog='game.py')
   >>> parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
   >>> parser.parse_args(['rock'])
   Namespace(move='rock')
   >>> parser.parse_args(['fire'])
   usage: game.py [-h] {rock,paper,scissors}
   game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',
   'paper', 'scissors')

Note that inclusion in the *choices* container is checked after any
type conversions have been performed, so the type of the objects in
the *choices* container should match the type specified:

   >>> parser = argparse.ArgumentParser(prog='doors.py')
   >>> parser.add_argument('door', type=int, choices=range(1, 4))
   >>> print(parser.parse_args(['3']))
   Namespace(door=3)
   >>> parser.parse_args(['4'])
   usage: doors.py [-h] {1,2,3}
   doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)

Any object that supports the "in" operator can be passed as the
*choices* value, so "dict" objects, "set" objects, custom containers,
etc. are all supported.


16.4.3.8. required
------------------

In general, the "argparse" module assumes that flags like "-f" and "--
bar" indicate *optional* arguments, which can always be omitted at the
command line. To make an option *required*, "True" can be specified
for the "required=" keyword argument to "add_argument()":

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', required=True)
   >>> parser.parse_args(['--foo', 'BAR'])
   Namespace(foo='BAR')
   >>> parser.parse_args([])
   usage: argparse.py [-h] [--foo FOO]
   argparse.py: error: option --foo is required

As the example shows, if an option is marked as "required",
"parse_args()" will report an error if that option is not present at
the command line.

注解: Required options are generally considered bad form because
  users expect *options* to be *optional*, and thus they should be
  avoided when possible.


16.4.3.9. help
--------------

The "help" value is a string containing a brief description of the
argument. When a user requests help (usually by using "-h" or "--help"
at the command line), these "help" descriptions will be displayed with
each argument:

   >>> parser = argparse.ArgumentParser(prog='frobble')
   >>> parser.add_argument('--foo', action='store_true',
   ...                     help='foo the bars before frobbling')
   >>> parser.add_argument('bar', nargs='+',
   ...                     help='one of the bars to be frobbled')
   >>> parser.parse_args(['-h'])
   usage: frobble [-h] [--foo] bar [bar ...]

   positional arguments:
    bar     one of the bars to be frobbled

   optional arguments:
    -h, --help  show this help message and exit
    --foo   foo the bars before frobbling

The "help" strings can include various format specifiers to avoid
repetition of things like the program name or the argument default.
The available specifiers include the program name, "%(prog)s" and most
keyword arguments to "add_argument()", e.g. "%(default)s", "%(type)s",
etc.:

   >>> parser = argparse.ArgumentParser(prog='frobble')
   >>> parser.add_argument('bar', nargs='?', type=int, default=42,
   ...                     help='the bar to %(prog)s (default: %(default)s)')
   >>> parser.print_help()
   usage: frobble [-h] [bar]

   positional arguments:
    bar     the bar to frobble (default: 42)

   optional arguments:
    -h, --help  show this help message and exit

As the help string supports %-formatting, if you want a literal "%" to
appear in the help string, you must escape it as "%%".

"argparse" supports silencing the help entry for certain options, by
setting the "help" value to "argparse.SUPPRESS":

   >>> parser = argparse.ArgumentParser(prog='frobble')
   >>> parser.add_argument('--foo', help=argparse.SUPPRESS)
   >>> parser.print_help()
   usage: frobble [-h]

   optional arguments:
     -h, --help  show this help message and exit


16.4.3.10. metavar
------------------

When "ArgumentParser" generates help messages, it needs some way to
refer to each expected argument.  By default, ArgumentParser objects
use the dest value as the "name" of each object.  By default, for
positional argument actions, the dest value is used directly, and for
optional argument actions, the dest value is uppercased.  So, a single
positional argument with "dest='bar'" will be referred to as "bar". A
single optional argument "--foo" that should be followed by a single
command-line argument will be referred to as "FOO".  An example:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> parser.add_argument('bar')
   >>> parser.parse_args('X --foo Y'.split())
   Namespace(bar='X', foo='Y')
   >>> parser.print_help()
   usage:  [-h] [--foo FOO] bar

   positional arguments:
    bar

   optional arguments:
    -h, --help  show this help message and exit
    --foo FOO

An alternative name can be specified with "metavar":

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', metavar='YYY')
   >>> parser.add_argument('bar', metavar='XXX')
   >>> parser.parse_args('X --foo Y'.split())
   Namespace(bar='X', foo='Y')
   >>> parser.print_help()
   usage:  [-h] [--foo YYY] XXX

   positional arguments:
    XXX

   optional arguments:
    -h, --help  show this help message and exit
    --foo YYY

Note that "metavar" only changes the *displayed* name - the name of
the attribute on the "parse_args()" object is still determined by the
dest value.

Different values of "nargs" may cause the metavar to be used multiple
times. Providing a tuple to "metavar" specifies a different display
for each of the arguments:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x', nargs=2)
   >>> parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz'))
   >>> parser.print_help()
   usage: PROG [-h] [-x X X] [--foo bar baz]

   optional arguments:
    -h, --help     show this help message and exit
    -x X X
    --foo bar baz


16.4.3.11. dest
---------------

Most "ArgumentParser" actions add some value as an attribute of the
object returned by "parse_args()".  The name of this attribute is
determined by the "dest" keyword argument of "add_argument()".  For
positional argument actions, "dest" is normally supplied as the first
argument to "add_argument()":

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('bar')
   >>> parser.parse_args(['XXX'])
   Namespace(bar='XXX')

For optional argument actions, the value of "dest" is normally
inferred from the option strings.  "ArgumentParser" generates the
value of "dest" by taking the first long option string and stripping
away the initial "--" string.  If no long option strings were
supplied, "dest" will be derived from the first short option string by
stripping the initial "-" character.  Any internal "-" characters will
be converted to "_" characters to make sure the string is a valid
attribute name.  The examples below illustrate this behavior:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('-f', '--foo-bar', '--foo')
   >>> parser.add_argument('-x', '-y')
   >>> parser.parse_args('-f 1 -x 2'.split())
   Namespace(foo_bar='1', x='2')
   >>> parser.parse_args('--foo 1 -y 2'.split())
   Namespace(foo_bar='1', x='2')

"dest" allows a custom attribute name to be provided:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', dest='bar')
   >>> parser.parse_args('--foo XXX'.split())
   Namespace(bar='XXX')


16.4.3.12. Action classes
-------------------------

Action classes implement the Action API, a callable which returns a
callable which processes arguments from the command-line. Any object
which follows this API may be passed as the "action" parameter to
"add_argument()".

class argparse.Action(option_strings, dest, nargs=None, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None)

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 Action class must accept the two positional
arguments plus any keyword arguments passed to
"ArgumentParser.add_argument()" except for the "action" itself.

Instances of Action (or return value of any callable to the "action"
parameter) should have attributes "dest", "option_strings", "default",
"type", "required", "help", etc. defined. The easiest way to ensure
these attributes are defined is to call "Action.__init__".

Action instances should be callable, so subclasses must override the
"__call__" method, which should accept four parameters:

* "parser" - The ArgumentParser object which contains this action.

* "namespace" - The "Namespace" object that will be returned by
  "parse_args()".  Most actions add an attribute to this object using
  "setattr()".

* "values" - The associated command-line arguments, with any type
  conversions applied.  Type conversions are specified with the type
  keyword argument to "add_argument()".

* "option_string" - The option string that was used to invoke this
  action. The "option_string" argument is optional, and will be absent
  if the action is associated with a positional argument.

The "__call__" method may perform arbitrary actions, but will
typically set attributes on the "namespace" based on "dest" and
"values".


16.4.4. The parse_args() method
===============================

ArgumentParser.parse_args(args=None, namespace=None)

   Convert argument strings to objects and assign them as attributes
   of the namespace.  Return the populated namespace.

   Previous calls to "add_argument()" determine exactly what objects
   are created and how they are assigned. See the documentation for
   "add_argument()" for details.

   * args - List of strings to parse.  The default is taken from
     "sys.argv".

   * namespace - An object to take the attributes.  The default is a
     new empty "Namespace" object.


16.4.4.1. Option value syntax
-----------------------------

The "parse_args()" method supports several ways of specifying the
value of an option (if it takes one).  In the simplest case, the
option and its value are passed as two separate arguments:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x')
   >>> parser.add_argument('--foo')
   >>> parser.parse_args(['-x', 'X'])
   Namespace(foo=None, x='X')
   >>> parser.parse_args(['--foo', 'FOO'])
   Namespace(foo='FOO', x=None)

For long options (options with names longer than a single character),
the option and value can also be passed as a single command-line
argument, using "=" to separate them:

   >>> parser.parse_args(['--foo=FOO'])
   Namespace(foo='FOO', x=None)

For short options (options only one character long), the option and
its value can be concatenated:

   >>> parser.parse_args(['-xX'])
   Namespace(foo=None, x='X')

Several short options can be joined together, using only a single "-"
prefix, as long as only the last option (or none of them) requires a
value:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x', action='store_true')
   >>> parser.add_argument('-y', action='store_true')
   >>> parser.add_argument('-z')
   >>> parser.parse_args(['-xyzZ'])
   Namespace(x=True, y=True, z='Z')


16.4.4.2. Invalid arguments
---------------------------

While parsing the command line, "parse_args()" checks for a variety of
errors, including ambiguous options, invalid types, invalid options,
wrong number of positional arguments, etc.  When it encounters such an
error, it exits and prints the error along with a usage message:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('--foo', type=int)
   >>> parser.add_argument('bar', nargs='?')

   >>> # invalid type
   >>> parser.parse_args(['--foo', 'spam'])
   usage: PROG [-h] [--foo FOO] [bar]
   PROG: error: argument --foo: invalid int value: 'spam'

   >>> # invalid option
   >>> parser.parse_args(['--bar'])
   usage: PROG [-h] [--foo FOO] [bar]
   PROG: error: no such option: --bar

   >>> # wrong number of arguments
   >>> parser.parse_args(['spam', 'badger'])
   usage: PROG [-h] [--foo FOO] [bar]
   PROG: error: extra arguments found: badger


16.4.4.3. Arguments containing "-"
----------------------------------

The "parse_args()" method attempts to give errors whenever the user
has clearly made a mistake, but some situations are inherently
ambiguous.  For example, the command-line argument "-1" could either
be an attempt to specify an option or an attempt to provide a
positional argument. The "parse_args()" method is cautious here:
positional arguments may only begin with "-" if they look like
negative numbers and there are no options in the parser that look like
negative numbers:

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-x')
   >>> parser.add_argument('foo', nargs='?')

   >>> # no negative number options, so -1 is a positional argument
   >>> parser.parse_args(['-x', '-1'])
   Namespace(foo=None, x='-1')

   >>> # no negative number options, so -1 and -5 are positional arguments
   >>> parser.parse_args(['-x', '-1', '-5'])
   Namespace(foo='-5', x='-1')

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-1', dest='one')
   >>> parser.add_argument('foo', nargs='?')

   >>> # negative number options present, so -1 is an option
   >>> parser.parse_args(['-1', 'X'])
   Namespace(foo=None, one='X')

   >>> # negative number options present, so -2 is an option
   >>> parser.parse_args(['-2'])
   usage: PROG [-h] [-1 ONE] [foo]
   PROG: error: no such option: -2

   >>> # negative number options present, so both -1s are options
   >>> parser.parse_args(['-1', '-1'])
   usage: PROG [-h] [-1 ONE] [foo]
   PROG: error: argument -1: expected one argument

If you have positional arguments that must begin with "-" and don't
look like negative numbers, you can insert the pseudo-argument "'--'"
which tells "parse_args()" that everything after that is a positional
argument:

   >>> parser.parse_args(['--', '-f'])
   Namespace(foo='-f', one=None)


16.4.4.4. Argument abbreviations (prefix matching)
--------------------------------------------------

The "parse_args()" method by default allows long options to be
abbreviated to a prefix, if the abbreviation is unambiguous (the
prefix matches a unique option):

   >>> parser = argparse.ArgumentParser(prog='PROG')
   >>> parser.add_argument('-bacon')
   >>> parser.add_argument('-badger')
   >>> parser.parse_args('-bac MMM'.split())
   Namespace(bacon='MMM', badger=None)
   >>> parser.parse_args('-bad WOOD'.split())
   Namespace(bacon=None, badger='WOOD')
   >>> parser.parse_args('-ba BA'.split())
   usage: PROG [-h] [-bacon BACON] [-badger BADGER]
   PROG: error: ambiguous option: -ba could match -badger, -bacon

An error is produced for arguments that could produce more than one
options. This feature can be disabled by setting allow_abbrev to
"False".


16.4.4.5. Beyond "sys.argv"
---------------------------

Sometimes it may be useful to have an ArgumentParser parse arguments
other than those of "sys.argv".  This can be accomplished by passing a
list of strings to "parse_args()".  This is useful for testing at the
interactive prompt:

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument(
   ...     'integers', metavar='int', type=int, choices=range(10),
   ...     nargs='+', help='an integer in the range 0..9')
   >>> parser.add_argument(
   ...     '--sum', dest='accumulate', action='store_const', const=sum,
   ...     default=max, help='sum the integers (default: find the max)')
   >>> parser.parse_args(['1', '2', '3', '4'])
   Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])
   >>> parser.parse_args(['1', '2', '3', '4', '--sum'])
   Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])


16.4.4.6. The Namespace object
------------------------------

class argparse.Namespace

   Simple class used by default by "parse_args()" to create an object
   holding attributes and return it.

This class is deliberately simple, just an "object" subclass with a
readable string representation. If you prefer to have dict-like view
of the attributes, you can use the standard Python idiom, "vars()":

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> args = parser.parse_args(['--foo', 'BAR'])
   >>> vars(args)
   {'foo': 'BAR'}

It may also be useful to have an "ArgumentParser" assign attributes to
an already existing object, rather than a new "Namespace" object.
This can be achieved by specifying the "namespace=" keyword argument:

   >>> class C:
   ...     pass
   ...
   >>> c = C()
   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo')
   >>> parser.parse_args(args=['--foo', 'BAR'], namespace=c)
   >>> c.foo
   'BAR'


16.4.5. Other utilities
=======================


16.4.5.1. Sub-commands
----------------------

ArgumentParser.add_subparsers([title][, description][, prog][, parser_class][, action][, option_string][, dest][, help][, metavar])

   Many programs split up their functionality into a number of sub-
   commands, for example, the "svn" program can invoke sub-commands
   like "svn checkout", "svn update", and "svn commit".  Splitting up
   functionality this way can be a particularly good idea when a
   program performs several different functions which require
   different kinds of command-line arguments. "ArgumentParser"
   supports the creation of such sub-commands with the
   "add_subparsers()" method.  The "add_subparsers()" method is
   normally called with no arguments and returns a special action
   object.  This object has a single method, "add_parser()", which
   takes a command name and any "ArgumentParser" constructor
   arguments, and returns an "ArgumentParser" object that can be
   modified as usual.

   Description of parameters:

   * title - title for the sub-parser group in help output; by
     default "subcommands" if description is provided, otherwise uses
     title for positional arguments

   * description - description for the sub-parser group in help
     output, by default "None"

   * prog - usage information that will be displayed with sub-
     command help, by default the name of the program and any
     positional arguments before the subparser argument

   * parser_class - class which will be used to create sub-parser
     instances, by default the class of the current parser (e.g.
     ArgumentParser)

   * action - the basic type of action to be taken when this
     argument is encountered at the command line

   * dest - name of the attribute under which sub-command name will
     be stored; by default "None" and no value is stored

   * help - help for sub-parser group in help output, by default
     "None"

   * metavar - string presenting available sub-commands in help; by
     default it is "None" and presents sub-commands in form {cmd1,
     cmd2, ..}

   Some example usage:

      >>> # create the top-level parser
      >>> parser = argparse.ArgumentParser(prog='PROG')
      >>> parser.add_argument('--foo', action='store_true', help='foo help')
      >>> subparsers = parser.add_subparsers(help='sub-command help')
      >>>
      >>> # create the parser for the "a" command
      >>> parser_a = subparsers.add_parser('a', help='a help')
      >>> parser_a.add_argument('bar', type=int, help='bar help')
      >>>
      >>> # create the parser for the "b" command
      >>> parser_b = subparsers.add_parser('b', help='b help')
      >>> parser_b.add_argument('--baz', choices='XYZ', help='baz help')
      >>>
      >>> # parse some argument lists
      >>> parser.parse_args(['a', '12'])
      Namespace(bar=12, foo=False)
      >>> parser.parse_args(['--foo', 'b', '--baz', 'Z'])
      Namespace(baz='Z', foo=True)

   Note that the object returned by "parse_args()" will only contain
   attributes for the main parser and the subparser that was selected
   by the command line (and not any other subparsers).  So in the
   example above, when the "a" command is specified, only the "foo"
   and "bar" attributes are present, and when the "b" command is
   specified, only the "foo" and "baz" attributes are present.

   Similarly, when a help message is requested from a subparser, only
   the help for that particular parser will be printed.  The help
   message will not include parent parser or sibling parser messages.
   (A help message for each subparser command, however, can be given
   by supplying the "help=" argument to "add_parser()" as above.)

      >>> parser.parse_args(['--help'])
      usage: PROG [-h] [--foo] {a,b} ...

      positional arguments:
        {a,b}   sub-command help
          a     a help
          b     b help

      optional arguments:
        -h, --help  show this help message and exit
        --foo   foo help

      >>> parser.parse_args(['a', '--help'])
      usage: PROG a [-h] bar

      positional arguments:
        bar     bar help

      optional arguments:
        -h, --help  show this help message and exit

      >>> parser.parse_args(['b', '--help'])
      usage: PROG b [-h] [--baz {X,Y,Z}]

      optional arguments:
        -h, --help     show this help message and exit
        --baz {X,Y,Z}  baz help

   The "add_subparsers()" method also supports "title" and
   "description" keyword arguments.  When either is present, the
   subparser's commands will appear in their own group in the help
   output.  For example:

      >>> parser = argparse.ArgumentParser()
      >>> subparsers = parser.add_subparsers(title='subcommands',
      ...                                    description='valid subcommands',
      ...                                    help='additional help')
      >>> subparsers.add_parser('foo')
      >>> subparsers.add_parser('bar')
      >>> parser.parse_args(['-h'])
      usage:  [-h] {foo,bar} ...

      optional arguments:
        -h, --help  show this help message and exit

      subcommands:
        valid subcommands

        {foo,bar}   additional help

   Furthermore, "add_parser" supports an additional "aliases"
   argument, which allows multiple strings to refer to the same
   subparser. This example, like "svn", aliases "co" as a shorthand
   for "checkout":

      >>> parser = argparse.ArgumentParser()
      >>> subparsers = parser.add_subparsers()
      >>> checkout = subparsers.add_parser('checkout', aliases=['co'])
      >>> checkout.add_argument('foo')
      >>> parser.parse_args(['co', 'bar'])
      Namespace(foo='bar')

   One particularly effective way of handling sub-commands is to
   combine the use of the "add_subparsers()" method with calls to
   "set_defaults()" so that each subparser knows which Python function
   it should execute.  For example:

      >>> # sub-command functions
      >>> def foo(args):
      ...     print(args.x * args.y)
      ...
      >>> def bar(args):
      ...     print('((%s))' % args.z)
      ...
      >>> # create the top-level parser
      >>> parser = argparse.ArgumentParser()
      >>> subparsers = parser.add_subparsers()
      >>>
      >>> # create the parser for the "foo" command
      >>> parser_foo = subparsers.add_parser('foo')
      >>> parser_foo.add_argument('-x', type=int, default=1)
      >>> parser_foo.add_argument('y', type=float)
      >>> parser_foo.set_defaults(func=foo)
      >>>
      >>> # create the parser for the "bar" command
      >>> parser_bar = subparsers.add_parser('bar')
      >>> parser_bar.add_argument('z')
      >>> parser_bar.set_defaults(func=bar)
      >>>
      >>> # parse the args and call whatever function was selected
      >>> args = parser.parse_args('foo 1 -x 2'.split())
      >>> args.func(args)
      2.0
      >>>
      >>> # parse the args and call whatever function was selected
      >>> args = parser.parse_args('bar XYZYX'.split())
      >>> args.func(args)
      ((XYZYX))

   This way, you can let "parse_args()" do the job of calling the
   appropriate function after argument parsing is complete.
   Associating functions with actions like this is typically the
   easiest way to handle the different actions for each of your
   subparsers.  However, if it is necessary to check the name of the
   subparser that was invoked, the "dest" keyword argument to the
   "add_subparsers()" call will work:

      >>> parser = argparse.ArgumentParser()
      >>> subparsers = parser.add_subparsers(dest='subparser_name')
      >>> subparser1 = subparsers.add_parser('1')
      >>> subparser1.add_argument('-x')
      >>> subparser2 = subparsers.add_parser('2')
      >>> subparser2.add_argument('y')
      >>> parser.parse_args(['2', 'frobble'])
      Namespace(subparser_name='2', y='frobble')


16.4.5.2. FileType objects
--------------------------

class argparse.FileType(mode='r', bufsize=-1, encoding=None, errors=None)

   The "FileType" factory creates objects that can be passed to the
   type argument of "ArgumentParser.add_argument()".  Arguments that
   have "FileType" objects as their type will open command-line
   arguments as files with the requested modes, buffer sizes,
   encodings and error handling (see the "open()" function for more
   details):

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('--raw', type=argparse.FileType('wb', 0))
      >>> parser.add_argument('out', type=argparse.FileType('w', encoding='UTF-8'))
      >>> parser.parse_args(['--raw', 'raw.dat', 'file.txt'])
      Namespace(out=<_io.TextIOWrapper name='file.txt' mode='w' encoding='UTF-8'>, raw=<_io.FileIO name='raw.dat' mode='wb'>)

   FileType objects understand the pseudo-argument "'-'" and
   automatically convert this into "sys.stdin" for readable "FileType"
   objects and "sys.stdout" for writable "FileType" objects:

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('infile', type=argparse.FileType('r'))
      >>> parser.parse_args(['-'])
      Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)

   3.4 新版功能: The *encodings* and *errors* keyword arguments.


16.4.5.3. Argument groups
-------------------------

ArgumentParser.add_argument_group(title=None, description=None)

   By default, "ArgumentParser" groups command-line arguments into
   "positional arguments" and "optional arguments" when displaying
   help messages. When there is a better conceptual grouping of
   arguments than this default one, appropriate groups can be created
   using the "add_argument_group()" method:

      >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
      >>> group = parser.add_argument_group('group')
      >>> group.add_argument('--foo', help='foo help')
      >>> group.add_argument('bar', help='bar help')
      >>> parser.print_help()
      usage: PROG [--foo FOO] bar

      group:
        bar    bar help
        --foo FOO  foo help

   The "add_argument_group()" method returns an argument group object
   which has an "add_argument()" method just like a regular
   "ArgumentParser".  When an argument is added to the group, the
   parser treats it just like a normal argument, but displays the
   argument in a separate group for help messages.  The
   "add_argument_group()" method accepts *title* and *description*
   arguments which can be used to customize this display:

      >>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
      >>> group1 = parser.add_argument_group('group1', 'group1 description')
      >>> group1.add_argument('foo', help='foo help')
      >>> group2 = parser.add_argument_group('group2', 'group2 description')
      >>> group2.add_argument('--bar', help='bar help')
      >>> parser.print_help()
      usage: PROG [--bar BAR] foo

      group1:
        group1 description

        foo    foo help

      group2:
        group2 description

        --bar BAR  bar help

   Note that any arguments not in your user-defined groups will end up
   back in the usual "positional arguments" and "optional arguments"
   sections.


16.4.5.4. Mutual exclusion
--------------------------

ArgumentParser.add_mutually_exclusive_group(required=False)

   Create a mutually exclusive group. "argparse" will make sure that
   only one of the arguments in the mutually exclusive group was
   present on the command line:

      >>> parser = argparse.ArgumentParser(prog='PROG')
      >>> group = parser.add_mutually_exclusive_group()
      >>> group.add_argument('--foo', action='store_true')
      >>> group.add_argument('--bar', action='store_false')
      >>> parser.parse_args(['--foo'])
      Namespace(bar=True, foo=True)
      >>> parser.parse_args(['--bar'])
      Namespace(bar=False, foo=False)
      >>> parser.parse_args(['--foo', '--bar'])
      usage: PROG [-h] [--foo | --bar]
      PROG: error: argument --bar: not allowed with argument --foo

   The "add_mutually_exclusive_group()" method also accepts a
   *required* argument, to indicate that at least one of the mutually
   exclusive arguments is required:

      >>> parser = argparse.ArgumentParser(prog='PROG')
      >>> group = parser.add_mutually_exclusive_group(required=True)
      >>> group.add_argument('--foo', action='store_true')
      >>> group.add_argument('--bar', action='store_false')
      >>> parser.parse_args([])
      usage: PROG [-h] (--foo | --bar)
      PROG: error: one of the arguments --foo --bar is required

   Note that currently mutually exclusive argument groups do not
   support the *title* and *description* arguments of
   "add_argument_group()".


16.4.5.5. Parser defaults
-------------------------

ArgumentParser.set_defaults(**kwargs)

   Most of the time, the attributes of the object returned by
   "parse_args()" will be fully determined by inspecting the command-
   line arguments and the argument actions.  "set_defaults()" allows
   some additional attributes that are determined without any
   inspection of the command line to be added:

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('foo', type=int)
      >>> parser.set_defaults(bar=42, baz='badger')
      >>> parser.parse_args(['736'])
      Namespace(bar=42, baz='badger', foo=736)

   Note that parser-level defaults always override argument-level
   defaults:

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('--foo', default='bar')
      >>> parser.set_defaults(foo='spam')
      >>> parser.parse_args([])
      Namespace(foo='spam')

   Parser-level defaults can be particularly useful when working with
   multiple parsers.  See the "add_subparsers()" method for an example
   of this type.

ArgumentParser.get_default(dest)

   Get the default value for a namespace attribute, as set by either
   "add_argument()" or by "set_defaults()":

      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('--foo', default='badger')
      >>> parser.get_default('foo')
      'badger'


16.4.5.6. Printing help
-----------------------

In most typical applications, "parse_args()" will take care of
formatting and printing any usage or error messages.  However, several
formatting methods are available:

ArgumentParser.print_usage(file=None)

   Print a brief description of how the "ArgumentParser" should be
   invoked on the command line.  If *file* is "None", "sys.stdout" is
   assumed.

ArgumentParser.print_help(file=None)

   Print a help message, including the program usage and information
   about the arguments registered with the "ArgumentParser".  If
   *file* is "None", "sys.stdout" is assumed.

There are also variants of these methods that simply return a string
instead of printing it:

ArgumentParser.format_usage()

   Return a string containing a brief description of how the
   "ArgumentParser" should be invoked on the command line.

ArgumentParser.format_help()

   Return a string containing a help message, including the program
   usage and information about the arguments registered with the
   "ArgumentParser".


16.4.5.7. Partial parsing
-------------------------

ArgumentParser.parse_known_args(args=None, namespace=None)

Sometimes a script may only parse a few of the command-line arguments,
passing the remaining arguments on to another script or program. In
these cases, the "parse_known_args()" method can be useful.  It works
much like "parse_args()" except that it does not produce an error when
extra arguments are present.  Instead, it returns a two item tuple
containing the populated namespace and the list of remaining argument
strings.

   >>> parser = argparse.ArgumentParser()
   >>> parser.add_argument('--foo', action='store_true')
   >>> parser.add_argument('bar')
   >>> parser.parse_known_args(['--foo', '--badger', 'BAR', 'spam'])
   (Namespace(bar='BAR', foo=True), ['--badger', 'spam'])

警告: Prefix matching rules apply to "parse_known_args()". The
  parser may consume an option even if it's just a prefix of one of
  its known options, instead of leaving it in the remaining arguments
  list.


16.4.5.8. Customizing file parsing
----------------------------------

ArgumentParser.convert_arg_line_to_args(arg_line)

   Arguments that are read from a file (see the
   *fromfile_prefix_chars* keyword argument to the "ArgumentParser"
   constructor) are read one argument per line.
   "convert_arg_line_to_args()" can be overridden for fancier reading.

   This method takes a single argument *arg_line* which is a string
   read from the argument file.  It returns a list of arguments parsed
   from this string. The method is called once per line read from the
   argument file, in order.

   A useful override of this method is one that treats each space-
   separated word as an argument.  The following example demonstrates
   how to do this:

      class MyArgumentParser(argparse.ArgumentParser):
          def convert_arg_line_to_args(self, arg_line):
              return arg_line.split()


16.4.5.9. Exiting methods
-------------------------

ArgumentParser.exit(status=0, message=None)

   This method terminates the program, exiting with the specified
   *status* and, if given, it prints a *message* before that.

ArgumentParser.error(message)

   This method prints a usage message including the *message* to the
   standard error and terminates the program with a status code of 2.


16.4.6. Upgrading optparse code
===============================

Originally, the "argparse" module had attempted to maintain
compatibility with "optparse".  However, "optparse" was difficult to
extend transparently, particularly with the changes required to
support the new "nargs=" specifiers and better usage messages.  When
most everything in "optparse" had either been copy-pasted over or
monkey-patched, it no longer seemed practical to try to maintain the
backwards compatibility.

The "argparse" module improves on the standard library "optparse"
module in a number of ways including:

* Handling positional arguments.

* Supporting sub-commands.

* Allowing alternative option prefixes like "+" and "/".

* Handling zero-or-more and one-or-more style arguments.

* Producing more informative usage messages.

* Providing a much simpler interface for custom "type" and "action".

A partial upgrade path from "optparse" to "argparse":

* Replace all "optparse.OptionParser.add_option()" calls with
  "ArgumentParser.add_argument()" calls.

* Replace "(options, args) = parser.parse_args()" with "args =
  parser.parse_args()" and add additional
  "ArgumentParser.add_argument()" calls for the positional arguments.
  Keep in mind that what was previously called "options", now in the
  "argparse" context is called "args".

* Replace "optparse.OptionParser.disable_interspersed_args()" by
  setting "nargs" of a positional argument to argparse.REMAINDER, or
  use "parse_known_args()" to collect unparsed argument strings in a
  separate list.

* Replace callback actions and the "callback_*" keyword arguments
  with "type" or "action" arguments.

* Replace string names for "type" keyword arguments with the
  corresponding type objects (e.g. int, float, complex, etc).

* Replace "optparse.Values" with "Namespace" and
  "optparse.OptionError" and "optparse.OptionValueError" with
  "ArgumentError".

* Replace strings with implicit arguments such as "%default" or
  "%prog" with the standard Python syntax to use dictionaries to
  format strings, that is, "%(default)s" and "%(prog)s".

* Replace the OptionParser constructor "version" argument with a
  call to "parser.add_argument('--version', action='version',
  version='<the version>')".
