Python argparse:组合“选择"、“nargs"和“默认"时键入不一致

问题描述

我有以下 python 程序:

I have the following python program:

#!/usr/bin/env python

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('arg', choices=['foo', 'bar', 'baz'], default='foo', nargs='*')

args = parser.parse_args()

print(args)

如果我这样调用程序:

./prog.py

输出是

Namespace(arg='foo')

但是如果我用 foo 作为参数调用程序:

But if I invoke the program with foo as an argument:

./prog.py foo

输出是

Namespace(arg=['foo'])

问题

如何让arg的默认值变成list?

我试过了

我试过设置 default=['foo'] 但结果是:

I've tried setting default=['foo'] but that results in:

prog.py: error: argument arg: invalid choice: ['foo'] (choose from 'foo', 'bar', 'baz')

推荐答案

这是一个旧的但开放的错误/问题的副本

This is a duplicate of an old, but open, bug/issue

http://bugs.python.org/issue9625(argparse:默认值问题对于使用选择时的变量 nargs)

带有 * 的 positional 得到一些特殊处理.如果你不提供值，它的默认值总是通过 choices 测试.

A positional with * gets some special handling. Its default is always passed through the choices test if you don't provide values.

将其与 optional

In [138]: p=argparse.ArgumentParser()
In [139]: a=p.add_argument('--arg',choices=['foo','bar','baz'],nargs='*')

In [140]: p.parse_args([])
Out[140]: Namespace(arg=None)
In [141]: a.default=['foo']
In [142]: p.parse_args([])
Out[142]: Namespace(arg=['foo'])

默认接受，无需测试:

In [143]: a.default=['xxx']
In [144]: p.parse_args([])
Out[144]: Namespace(arg=['xxx'])

相关代码为:

def _get_values(self, action, arg_strings):
    ...
    # when nargs='*' on a positional, if there were no command-line
    # args, use the default if it is anything other than None
    elif (not arg_strings and action.nargs == ZERO_OR_MORE and
          not action.option_strings):
        if action.default is not None:
            value = action.default
        else:
            value = arg_strings
        self._check_value(action, value)

提议的错误/问题补丁对此代码块进行了小幅更改.

The proposed bug/issue patch makes a small change to this block of code.

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