16.4. `argparse` — 用于命令行选项、自变量及子命令的剖析器 ¶

3.2 版新增。

此页面包含 API 参考信息。对于 Python 命令行剖析的更温和介绍，见 argparse 教程 .

The argparse 模块使编写用户友好的命令行接口变得容易。程序定义它要求什么自变量，和 argparse 会弄明白如何剖析出那些从 sys.argv 。 argparse 模块还自动生成帮助、用法消息及发出错误，当用户赋予程序无效自变量时。

16.4.1. 范例 ¶

以下代码是接收整数列表并产生 sum 或 max 的 Python 程序：

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))

假定将以上 Python 代码保存到文件称为 prog.py ，它可以运行在命令行并提供有用帮助消息：

$ 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)

当采用适当自变量运行时，它打印命令行整数的 sum 或 max：

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

若传入无效自变量，它会发出错误：

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

以下章节将带您完成此范例。

16.4.1.1. 创建剖析器 ¶

第 1 步使用 argparse 创建 ArgumentParser 对象：

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

The ArgumentParser object will hold all the information necessary to parse the command line into Python data types.

16.4.1.2. 添加自变量 ¶

Filling an ArgumentParser with information about program arguments is done by making calls to the add_argument() method. Generally, these calls tell the ArgumentParser how to take the strings on the command line and turn them into objects. This information is stored and used when parse_args() is called. For example:

>>> 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)')

Later, calling parse_args() will return an object with two attributes, integers and accumulate 。 integers attribute will be a list of one or more ints, and the accumulate attribute will be either the sum() function, if --sum was specified at the command line, or the max() function if it was not.

16.4.1.3. 剖析自变量 ¶

ArgumentParser parses arguments through the parse_args() method. This will inspect the command line, convert each argument to the appropriate type and then invoke the appropriate action. In most cases, this means a simple Namespace object will be built up from attributes parsed out of the command line:

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

In a script, parse_args() will typically be called with no arguments, and the ArgumentParser will automatically determine the command-line arguments from sys.argv .

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 ) ¶

创建新的 ArgumentParser object. All parameters should be passed as keyword arguments. Each parameter has its own more detailed description below, but in short they are:

prog - 程序的名称 (默认： sys.argv[0] )
usage - The string describing the program usage (default: generated from arguments added to parser)
description - Text to display before the argument help (default: none)
epilog - Text to display after the argument help (default: none)
parents - A list of ArgumentParser objects whose arguments should also be included
formatter_class - A class for customizing the help output
prefix_chars - The set of characters that prefix optional arguments (default: ‘-‘)
fromfile_prefix_chars - The set of characters that prefix files from which additional arguments should be read (default: None )
argument_default - The global default value for arguments (default: None )
conflict_handler - The strategy for resolving conflicting optionals (usually unnecessary)
add_help - Add a -h/–help option to the parser (default: True )

以下章节描述如何使用每个这些。

16.4.2.1. prog ¶

默认情况下， ArgumentParser objects uses sys.argv[0] to determine how to display the name of the program in help messages. This default is almost always desirable because it will make the help messages match how the program was invoked on the command line. For example, consider a file named myprogram.py 通过以下代码：

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

The help for this program will display myprogram.py as the program name (regardless of where the program was invoked from):

$ 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

To change this default behavior, another value can be supplied using the prog= 自变量对于 ArgumentParser :

>>> parser = argparse.ArgumentParser(prog='myprogram')
>>> parser.print_help()
usage: myprogram [-h]
optional arguments:
 -h, --help  show this help message and exit

Note that the program name, whether determined from sys.argv[0] or from the prog= argument, is available to help messages using the %(prog)s format specifier.

>>> 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 从其包含的自变量计算用法消息：

>>> 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

可以覆盖默认消息采用 usage= 关键词自变量：

>>> 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

The %(prog)s 格式说明符可用于在用法消息中填入程序名称。

16.4.2.3. description ¶

Most calls to the ArgumentParser constructor will use the description= keyword argument. This argument gives a brief description of what the program does and how it works. In help messages, the description is displayed between the command-line usage string and the help messages for the various arguments:

>>> 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 自变量。

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= 自变量对于 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

就像 description 自变量， epilog= text is by default line-wrapped, but this behavior can be adjusted with the formatter_class 自变量对于 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= 自变量对于 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.

注意

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

传递 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.

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= 自变量默认为 '-' . 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= 自变量默认为 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. conflict_handler ¶

ArgumentParser objects do not allow two actions with the same option string. By default, ArgumentParser objects raises 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= 自变量 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

注意， 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.

16.4.2.11. 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()

若 -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 作为 add_help= 自变量对于 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 - ，在这种情况下 -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. add_argument() 方法 ¶

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:

名称或标志 - 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 - 应消耗的命令行自变量数。
const - A constant value required by some action and nargs selections.
default - The value produced if the argument is absent from the 命令行。
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() .

以下章节描述如何使用每个这些。

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')

当 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: too few arguments

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() 。 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. (Note that the const keyword argument defaults to the rather unhelpful None .) 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'.split())
Namespace(foo=42)
								
```

'store_true' and 'store_false' - These store the values True and False respectively. These are special cases of 'store_const' 。例如：

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

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')

更多细节，见 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'])

注意， 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'.split())
Namespace(bar='XX', foo='YY')
>>> parser.parse_args('XX --foo'.split())
Namespace(bar='XX', foo='c')
>>> parser.parse_args(''.split())
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'.split())
Namespace(foo=['a', 'b'])
>>> parser.parse_args(''.split())
usage: PROG [-h] foo [foo ...]
PROG: error: too few arguments

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')

若 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 自变量 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:

当 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() 。见 action description for examples.
当 add_argument() is called with option strings (像 -f or --foo ) 和 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. 见 nargs description for examples.

The const keyword argument 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'.split())
Namespace(foo='2')
>>> parser.parse_args(''.split())
Namespace(foo=42)

若 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 等于 ? or * ， 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'.split())
Namespace(foo='a')
>>> parser.parse_args(''.split())
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. type ¶

默认情况下， 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 。 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 自变量：

>>> 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'.split())
Namespace(foo=9)
>>> parser.parse_args('7'.split())
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'.split())
Namespace(foo=7)
>>> parser.parse_args('11'.split())
usage: PROG [-h] {5,6,7,8,9}
PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)

见 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 对象， set objects, custom containers, etc. are all supported.

16.4.3.8. required ¶

一般而言， 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.

注意

Required options are generally considered bad form because users expect 选项到 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'.split())
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() ，如 %(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 ¶

当 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

注意， metavar only changes the displayed name - the name of the attribute on the parse_args() object is still determined by the dest 值。

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'.split())
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 参数用于 add_argument() .

class argparse. 动作 ( 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 本身。

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 对象使用 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 基于 dest and values .

16.4.4. parse_args() 方法 ¶

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() 了解细节。

By default, the argument strings are taken from sys.argv , and a new empty Namespace object is created for the attributes.

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'.split())
Namespace(foo=None, x='X')
>>> parser.parse_args('--foo FOO'.split())
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'.split())
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'.split())
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'.split())
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 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.

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'.split())
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= 关键词自变量：

>>> 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. 其它实用工具 ¶

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 ，和 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() 方法。 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.

参数的描述：

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, ..}

一些范例用法：

>>> # 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= 自变量对于 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() 方法还支持 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 对象：

>>> 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 版新增： The encodings and errors keyword arguments.

16.4.5.3. Argument groups ¶

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

默认情况下， 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() 方法：

>>> 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() 或通过 set_defaults() :

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

16.4.5.6. Printing help ¶

在大多数典型应用程序中， 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 ) ¶

ArgumentParser. print_help ( file=None ) ¶

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

ArgumentParser. format_usage ( ) ¶

ArgumentParser. format_help ( ) ¶

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

前缀匹配 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 ) ¶

16.4.5.9. Exiting methods ¶

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

ArgumentParser. error ( message ) ¶

16.4.6. 升级 optparse 代码 ¶

最初， 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:

处理位置自变量。
支持子命令。
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() 调用。
替换 (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 argparse context is called args .
Replace callback actions and the callback_* keyword arguments with type or action 自变量。
Replace string names for type keyword arguments with the corresponding type objects (e.g. int, float, complex, etc).
替换 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>') .

16.4. argparse — 用于命令行选项、自变量及子命令的剖析器 ¶