Argparse:如何处理可变数量的参数(nargs='*') [英] Argparse: how to handle variable number of arguments (nargs='*')

问题描述

我认为 nargs='*' 足以处理可变数量的参数.显然不是，我不明白这个错误的原因.

I thought that nargs='*' was enough to handle a variable number of arguments. Apparently it's not, and I don't understand the cause of this error.

代码:

p = argparse.ArgumentParser()
p.add_argument('pos')
p.add_argument('foo')
p.add_argument('--spam', default=24, type=int, dest='spam')
p.add_argument('vars', nargs='*')

p.parse_args('1 2 --spam 8 8 9'.split())

我认为生成的命名空间应该是 Namespace(pos='1', foo='2', spam='8', vars=['8', '9']).相反，argparse 给出了这个错误:

I think the resulting namespace should be Namespace(pos='1', foo='2', spam='8', vars=['8', '9']). Instead, argparse gives this error:

usage: prog.py [-h] [--spam SPAM] pos foo [vars [vars ...]]
error: unrecognized arguments: 9 8

基本上，argparse 不知道把这些额外的参数放在哪里......为什么会这样?

Basically, argparse doesn't know where to put those additional arguments... Why is that?

推荐答案

相关的 Python 错误是 Issue 15112.

The relevant Python bug is Issue 15112.

argparse: nargs='*' 如果前面有一个选项和另一个位置参数，则位置参数不接受任何项

argparse: nargs='*' positional argument doesn't accept any items if preceded by an option and another positional

当 argparse 解析 ['1', '2', '--spam', '8', '8', '9'] 它首先尝试匹配 ['1','2'] 使用尽可能多的位置参数.使用您的参数，模式匹配字符串为 AAA*:pos 和 foo 各有 1 个参数，vars(记住 * 表示零或更多).

When argparse parses ['1', '2', '--spam', '8', '8', '9'] it first tries to match ['1','2'] with as many of the positional arguments as possible. With your arguments the pattern matching string is AAA*: 1 argument each for pos and foo, and zero arguments for vars (remember * means ZERO_OR_MORE).

['--spam','8'] 由您的 --spam 参数处理.由于 vars 已经设置为 []，所以没有什么可以处理 ['8','9'].

['--spam','8'] are handled by your --spam argument. Since vars has already been set to [], there is nothing left to handle ['8','9'].

对argparse 的编程更改检查0 参数字符串是否满足模式的情况，但仍有optionals 需要解析.然后它推迟处理 * 参数.

The programming change to argparse checks for the case where 0 argument strings is satisfying the pattern, but there are still optionals to be parsed. It then defers the handling of that * argument.

您可能可以通过首先使用 parse_known_args，然后通过另一个对 parse_args.

You might be able to get around this by first parsing the input with parse_known_args, and then handling the remainder with another call to parse_args.

为了在位置之间穿插可选项有完全的自由，在 issue 14191 中，我建议使用 parse_known_args 只包含 optionals，后跟一个只知道位置的 parse_args.我在那里发布的 parse_intermixed_args 函数可以在 ArgumentParser 子类中实现，而无需修改 argparse.py 代码本身.

To have complete freedom in interspersing optionals among positionals, in issue 14191, I propose using parse_known_args with just the optionals, followed by a parse_args that only knows about the positionals. The parse_intermixed_args function that I posted there could be implemented in an ArgumentParser subclass, without modifying the argparse.py code itself.

这是一种处理子解析器的方法.我采用了 parse_known_intermixed_args 函数，简化了它的演示为了，然后使它成为解析器子类的 parse_known_args 函数.我不得不采取额外的步骤来避免递归.

Here's a way of handling subparsers. I've taken the parse_known_intermixed_args function, simplified it for presentation sake, and then made it the parse_known_args function of a Parser subclass. I had to take an extra step to avoid recursion.

最后我改变了子解析器动作的_parser_class，所以每个子解析器都使用这个替代的parse_known_args.另一种方法是将 _SubParsersAction 子类化，可能会修改其 __call__.

Finally I changed the _parser_class of the subparsers Action, so each subparser uses this alternative parse_known_args. An alternative would be to subclass _SubParsersAction, possibly modifying its __call__.

from argparse import ArgumentParser

def parse_known_intermixed_args(self, args=None, namespace=None):
    # self - argparse parser
    # simplified from http://bugs.python.org/file30204/test_intermixed.py
    parsefn = super(SubParser, self).parse_known_args # avoid recursion

    positionals = self._get_positional_actions()
    for action in positionals:
        # deactivate positionals
        action.save_nargs = action.nargs
        action.nargs = 0

    namespace, remaining_args = parsefn(args, namespace)
    for action in positionals:
        # remove the empty positional values from namespace
        if hasattr(namespace, action.dest):
            delattr(namespace, action.dest)
    for action in positionals:
        action.nargs = action.save_nargs
    # parse positionals
    namespace, extras = parsefn(remaining_args, namespace)
    return namespace, extras

class SubParser(ArgumentParser):
    parse_known_args = parse_known_intermixed_args

parser = ArgumentParser()
parser.add_argument('foo')
sp = parser.add_subparsers(dest='cmd')
sp._parser_class = SubParser # use different parser class for subparsers
spp1 = sp.add_parser('cmd1')
spp1.add_argument('-x')
spp1.add_argument('bar')
spp1.add_argument('vars',nargs='*')

print parser.parse_args('foo cmd1 bar -x one 8 9'.split())
# Namespace(bar='bar', cmd='cmd1', foo='foo', vars=['8', '9'], x='one')

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