Python argparse:组合"choices"，"nargs"和"default"时出现类型不一致 [英] Python argparse: type inconsistencies when combining 'choices', 'nargs' and 'default'

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

输出为

Namespace(arg='foo')

但是如果我使用 foo 作为参数来调用程序:

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

./prog.py foo

输出为

Namespace(arg=['foo'])

问题

如何获取 arg 的默认值以成为 list ?

How can I get arg's default value to become a list?

我已经尝试

我尝试设置 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

这是一个古老但开放的bug/问题的副本

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

http://bugs.python.org/issue9625 ( argparse:默认值问题用于使用选项时的变量nargs )

带有 * 的 positional 得到一些特殊的处理.如果您不提供值，则默认值始终通过 choices 测试传递.

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

将其与可选

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'])

默认值未经测试即被接受:

The default is accepted without testing:

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)

建议的错误/问题补丁对该代码块进行了少量更改.

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

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