python-2.7 - python : argparse throwing value error when combining positional and optional argument

Question

我正在尝试使用 python 中的 argparse 库来读取可选参数和必需参数。到目前为止，我正在这样做:

import argparse
parser = argparse.ArgumentParser(description='Cleanup Script for Folder')

parser.add_argument('PATH_TO_WORKDIR_ROOT', type=str, dest='PATH_TO_WORKDIR_ROOT', action='store', help='(absolute or) relative path to directory \n   to work on, e.g. "..\MyFolder\\"')
parser.add_argument('--PATH_TO_BACKUP_ROOT', type=str, dest='PATH_TO_BACKUP_ROOT', action='store', help='(absolute or) relative path to Backup-Directory \n   default is ".\BACKUP\"')

args = parser.parse_args()

现在我正在测试我的代码，它给了我一个我不明白的值错误:

$ python argparsetest.py --help
Traceback (most recent call last):  
File "argparsetest.py", line 5, in <module>
    parser.add_argument('PATH_TO_WORKDIR_ROOT', type=str, dest='PATH_TO_WORKDIR_ ROOT', action='store', help='(absolute or)
relative path to directory \n   to wo rk on, e.g. "..\MyFolder\\"')  
File "C:\Program
Files\Enthought\Canopy\App\appdata\canopy-1.3.0.1715.win-x86_
64\lib\argparse.py", line 1262, in add_argument
    raise ValueError('dest supplied twice for positional argument') ValueError: dest supplied twice for positional argument

只有一个位置参数，不是，目的地不同。我真的不明白麻烦:)

非常感谢!

Answer 1

查看中的以下代码argparse.py :

# =======================
# Adding argument actions
# =======================
def add_argument(self, *args, **kwargs):
    """
    add_argument(dest, ..., name=value, ...)
    add_argument(option_string, option_string, ..., name=value, ...)
    """

    # if no positional args are supplied or only one is supplied and
    # it doesn't look like an option string, parse a positional
    # argument
    chars = self.prefix_chars
    if not args or len(args) == 1 and args[0][0] not in chars:
        if args and 'dest' in kwargs:
            raise ValueError('dest supplied twice for positional argument')
        kwargs = self._get_positional_kwargs(*args, **kwargs)

由于您没有先于 --到 PATH_TO_WORKDIR_ROOT 它认为第一个参数是 dest因此，当您提供 dest 时会引发错误再次作为命名参数。