Argparse:在“ –help”中包含默认值的方法吗?

问题:Argparse:在“ –help”中包含默认值的方法吗?

假设我有以下argparse代码段:

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

当前,--help返回:

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

          Test the reliability/uptime of a connection.



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

我更喜欢这样的东西:

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

偷看帮助格式化程序代码,发现选择有限。是否有一种巧妙的方法来argparse--scan-time类似的方式打印默认值,还是我应该只是help格式化程序的子类?

Suppose I have the following argparse snippet:

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

Currently, --help returns:

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

          Test the reliability/uptime of a connection.



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

I would prefer something like:

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

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


回答 0

使用argparse.ArgumentDefaultsHelpFormatter格式化程序

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

引用文档:

另一个可用的格式化程序类ArgumentDefaultsHelpFormatter,将添加有关每个参数的默认值的信息。

请注意,这仅适用于已定义帮助文本的参数;如果没有help参数值,则没有帮助消息将有关默认值的信息添加到中

然后,您的扫描时间选项的确切输出将变为:

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

Use the argparse.ArgumentDefaultsHelpFormatter formatter:

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

To quote the documentation:

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

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

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

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

回答 1

添加'%(default)s'到帮助参数以控制显示内容。

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

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

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

回答 2

包装类

这是迄今为止我发现的最可靠,最干的方法,既可以显示默认值,也可以同时使用另一个格式化程序argparse.RawTextHelpFormatter

#!/usr/bin/env python3

import argparse

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

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

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

输出:

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

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

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

ArgumentDefaultsHelpFormatter+ RawTextHelpFormatter多重继承

多重继承可以正常工作,但它似乎不是公共API:

#!/usr/bin/env python3

import argparse

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

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

输出:

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

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

它之所以有效,是因为我们可以从https://github.com/python/cpython/blob/v3.6.5/Lib/argparse.py#L648的源代码中轻松地看到:

  • RawTextHelpFormatter 实施 _split_lines
  • ArgumentDefaultsHelpFormatter 实施 _get_help_string

因此我们可以猜测它们会一起正常工作。

但是,这似乎不是公共API,也不是的方法formatter_class,因此我认为目前尚无公共API方法。argparsedocstring说:

本模块中的所有其他类均视为实现细节。(还要注意,仅将HelpFormatter和RawDescriptionHelpFormatter视为对象名称是公共的-格式化程序对象的API仍被视为实现细节。)

另请参阅:自定义argparse帮助消息

在Python 3.6.5上测试。

Wrapper class

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

#!/usr/bin/env python3

import argparse

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

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

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

Output:

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

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

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

ArgumentDefaultsHelpFormatter + RawTextHelpFormatter multiple inheritance

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

#!/usr/bin/env python3

import argparse

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

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

Output:

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

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

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

  • RawTextHelpFormatter implements _split_lines
  • ArgumentDefaultsHelpFormatter implements _get_help_string

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

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

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

See also: Customize argparse help message

Tested on Python 3.6.5.