Argparse-不要用`nargs`捕获位置参数. [英] Argparse - do not catch positional arguments with `nargs`.

问题描述

我正在尝试编写一个函数wo，您可以通过argparse解析可变数量的参数-我知道我可以通过nargs="+"做到这一点.可悲的是，argparse帮助的工作方式(以及人们通常在CLI中编写自变量的方式)使位置自变量处于最后位置.这导致我的位置参数被视为可选参数的一部分.

I am trying to write a function wo which you can parse a variable amount of arguments via argparse - I know I can do this via nargs="+". Sadly, the way argparse help works (and the way people generally write arguments in the CLI) puts the positional arguments last. This leads to my positional argument being caught as part of the optional arguments.

#!/usr/bin/python
import argparse

parser = argparse.ArgumentParser()
parser.add_argument("positional", help="my positional arg", type=int)
parser.add_argument("-o", "--optional", help="my optional arg", nargs='+', type=float)
args = parser.parse_args()
print args.positional, args.optional

以./test.py -h身份运行将显示以下用法说明:

running this as ./test.py -h shows the following usage instruction:

usage: test.py [-h] [-o OPTIONAL [OPTIONAL ...]] positional

但是如果我运行./test.py -o 0.21 0.11 0.33 0.13 100给我

test.py: error: too few arguments

要正确解析args，我必须运行./test.py 100 -o 0.21 0.11 0.33 0.13

to get a correct parsing of args, I have to run ./test.py 100 -o 0.21 0.11 0.33 0.13

那我该怎么办

• 使argparse重新格式化用法输出，以减少误导或更好:

• make argparse reformat the usage output so that it is less misleading, OR, even better:

告诉argparse不能捕获可选参数-o的最后一个元素(如果它是列表中的最后一个元素)

tell argparse to not catch the last element for the optional argument -o if it is the last in the list

?

推荐答案

关于此的错误报告: http://bugs.python.org/issue9338

带有nargs ='?'，'*'或'+'的argparse可选参数后面不能跟位置

argparse optionals with nargs='?', '*' or '+' can't be followed by positionals

一个简单的(用户)修复方法是使用--将位置与可选位置分开:

A simple (user) fix is to use -- to separate postionals from optionals:

./test.py -o 0.21 0.11 0.33 0.13 -- 100

我写了一个补丁，其中保留了一些由位置使用的参数.但这不是一件小事.

I wrote a patch that reserves some of the arguments for use by the positional. But it isn't a trivial one.

关于更改使用线-最简单的方法是编写自己的用法，例如:

As for changing the usage line - the simplest thing is to write your own, e.g.:

usage: test.py [-h] positional [-o OPTIONAL [OPTIONAL ...]]
usage: test.py [-h] [-o OPTIONAL [OPTIONAL ...]] -- positional

我不建议在用法格式化程序中添加逻辑以进行此类更改.我认为这太复杂了.

I wouldn't recommend adding logic to the usage formatter to make this sort of change. I think it would get too complex.

另一个快速的解决方法是将此位置转换为(必需的)可选内容.它为用户提供了关于其订单的完全自由度，并可以减少混乱.如果您不想混淆"required optional"，则给它一个逻辑上的默认值.

Another quick fix is to turn this positional into an (required) optional. It gives the user complete freedom regarding their order, and might reduce confusion. If you don't want to confusion of a 'required optional' just give it a logical default.

usage: test.py [-h] [-o OPTIONAL [OPTIONAL ...]] -p POSITIONAL
usage: test.py [-h] [-o OPTIONAL [OPTIONAL ...]] [-p POS_WITH_DEFAULT]

---

对Help_Formatter的一个简单更改是，仅按定义的顺序列出参数.修改格式化程序行为的通常方法是将其子类化，并更改一种或两种方法.这些方法大多数都是专用"(_前缀)，因此您意识到将来的代码可能会(缓慢地)更改.

---

One easy change to the Help_Formatter is to simply list the arguments in the order that they are defined. The normal way of modifying formatter behavior is to subclass it, and change one or two methods. Most of these methods are 'private' (_ prefix), so you do so with the realization that future code might change (slowly).

在此方法中，actions是参数的定义顺序的列表.默认行为是从"positionals"中拆分"optionals"，然后将列表结尾处的位置重新组合在一起.还有其他代码可以处理需要换行的长行.通常，它将位置放在单独的行上.我已经省略了.

In this method, actions is the list of arguments, in the order in which they were defined. The default behavior is to split 'optionals' from 'positionals', and reassemble the list with positionals at the end. There's additional code that handles long lines that need wrapping. Normally it puts positionals on a separate line. I've omitted that.

class Formatter(argparse.HelpFormatter):
    # use defined argument order to display usage
    def _format_usage(self, usage, actions, groups, prefix):
        if prefix is None:
            prefix = 'usage: '

        # if usage is specified, use that
        if usage is not None:
            usage = usage % dict(prog=self._prog)

        # if no optionals or positionals are available, usage is just prog
        elif usage is None and not actions:
            usage = '%(prog)s' % dict(prog=self._prog)
        elif usage is None:
            prog = '%(prog)s' % dict(prog=self._prog)
            # build full usage string
            action_usage = self._format_actions_usage(actions, groups) # NEW
            usage = ' '.join([s for s in [prog, action_usage] if s])
            # omit the long line wrapping code
        # prefix with 'usage:'
        return '%s%s\n\n' % (prefix, usage)

parser = argparse.ArgumentParser(formatter_class=Formatter) 

哪个用法如下:

usage: stack26985650.py [-h] positional [-o OPTIONAL [OPTIONAL ...]]

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