Python，argparse：如何使 nargs=2 与 type=str 和 type=int

Python,argparse: how to have nargs=2 with type=str and type=int

提问by Bux31

I spent some times on the argparse documentation, but I'm still struggling with this module for one option in my program:

我花了一些时间在 argparse 文档上，但我仍然在用这个模块为我的程序中的一个选项而苦苦挣扎：

parser.add_argument("-r", "--rmsd", dest="rmsd", nargs=2,
    help="extract the poses that are close from a ref according RMSD",
    metavar=("ref","rmsd"))

I'd like to the first argument to be a string (type str) and mandatory, while the second argument should have type int, and if no value is given have a default one (let's say default=50). I know how to do that when there is only one argument expected, but I have no idea how to proceed when nargs=2... Is that even possible?

我希望第一个参数是字符串（type str）并且是强制性的，而第二个参数应该有 type int，如果没有给出值，则有一个默认值（比方说default=50）。我知道当只有一个参数时该怎么做，但是当 nargs=2 时我不知道如何继续......这可能吗？

回答by Mike Müller

I would recommend using two arguments:

我建议使用两个参数：

import argparse

parser = argparse.ArgumentParser(description='Example with to arguments.')

parser.add_argument('-r', '--ref', dest='reference', required=True,
                    help='be helpful')
parser.add_argument('-m', '--rmsd', type=int, dest='reference_msd',
                    default=50, help='be helpful')

args = parser.parse_args()
print args.reference
print args.reference_msd

回答by ashokadhikari

You can do the following. The requiredkeyword sets the field mandatory and the default=50sets the default value of the option to 50 if not specified:

您可以执行以下操作。该required关键字设置强制性领域和default=50将选项设置为50的默认值，如果没有指定：

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("-s", "--string", type=str, required=True)
parser.add_argument("-i", "--integer", type=int, default=50)

args = parser.parse_args()    
print args.string
print args.integer

Output:

输出：

$ python arg_parser.py -s test_string
    test_string
    50
$ python arg_parser.py -s test_string -i 100
    test_string
    100
$ python arg_parser.py -i 100
    usage: arg_parser.py [-h] -s STRING [-i INTEGER]
    arg_parser.py: error: argument -s/--string is required

回答by ashokadhikari

I tend to agree with Mike's solution, but here's another way. It's not ideal, since the usage/help string tells the user to use 1 or more arguments.

我倾向于同意迈克的解决方案，但这是另一种方式。这并不理想，因为用法/帮助字符串告诉用户使用 1 个或多个参数。

import argparse

def string_integer(int_default):
    """Action for argparse that allows a mandatory and optional
    argument, a string and integer, with a default for the integer.

    This factory function returns an Action subclass that is
    configured with the integer default.
    """
    class StringInteger(argparse.Action):
        """Action to assign a string and optional integer"""
        def __call__(self, parser, namespace, values, option_string=None):
            message = ''
            if len(values) not in [1, 2]:
                message = 'argument "{}" requires 1 or 2 arguments'.format(
                    self.dest)
            if len(values) == 2:
                try:
                    values[1] = int(values[1])
                except ValueError:
                    message = ('second argument to "{}" requires '
                               'an integer'.format(self.dest))
            else:
                values.append(int_default)
            if message:
                raise argparse.ArgumentError(self, message)            
            setattr(namespace, self.dest, values)
    return StringInteger

And with that, you get:

有了这个，你会得到：

>>> import argparse
>>> parser = argparse.ArgumentParser(description="")
parser.add_argument('-r', '--rmsd', dest='rmsd', nargs='+',
...                         action=string_integer(50),
...                         help="extract the poses that are close from a ref "
...                         "according RMSD")
>>> parser.parse_args('-r reference'.split())
Namespace(rmsd=['reference', 50])
>>> parser.parse_args('-r reference 30'.split())
Namespace(rmsd=['reference', 30])
>>> parser.parse_args('-r reference 30 3'.split())
usage: [-h] [-r RMSD [RMSD ...]]
: error: argument -r/--rmsd: argument "rmsd" requires 1 or 2 arguments
>>> parser.parse_args('-r reference 30.3'.split())
usage: [-h] [-r RMSD [RMSD ...]]
: error: argument -r/--rmsd: second argument to "rmsd" requires an integer

回答by Grant Mills

Sorry for jumping in way late. I'd use a function for type to call.

抱歉迟到了。我会使用一个函数来调用类型。

def two_args_str_int(x):
    try:
        return int(x)
    except:
        return x

parser.add_argument("-r", "--rmsd", dest="rmsd", nargs=2, type=two_args_str_int
    help="extract the poses that are close from a ref according RMSD",
    metavar=("ref","rmsd"))

回答by MarSoft

I had a similar problem, but "use two arguments" approach didn't work for me because I need a list of pairs: parser.add_argument('--replace', nargs=2, action='append')and if I use separate arguments then I would have to validate lengths of lists etc. Here is what I did:

我有一个类似的问题，但“使用两个参数”方法对我不起作用，因为我需要一个对列表：parser.add_argument('--replace', nargs=2, action='append')如果我使用单独的参数，那么我将不得不验证列表的长度等。这是我所做的：

1. Use tuplefor metavarto properly show help: tuple=('OLD', 'NEW')results in the help string being displayed as --replace OLD NEW. It is documented but I could not find it until tried different options.
2. Use custom validation: after parse_args, validate the resulting list's items and call parser.error()if something is wrong. That's because they have different data types.

1. 使用tuple了metavar正确显示的帮助：tuple=('OLD', 'NEW')在帮助字符串结果被显示为--replace OLD NEW。它被记录在案，但在尝试不同的选项之前我找不到它。
2. 使用自定义验证： after parse_args，验证结果列表的项目并在出现问题时调用parser.error()。那是因为它们具有不同的数据类型。