Multiple positional arguments with Python and argparse

Question

I’m trying to use argparse to parse the command line arguments for a program I’m working on. Essentially, I need to support multiple positional arguments spread within the optional arguments, but cannot get argparse to work in this situation. In the actual program, I’m using a custom action (I need to store a snapshot of the namespace each time a positional argument is found), but the problem I’m having can be replicated with the append action:

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-a', action='store_true')
>>> parser.add_argument('-b', action='store_true')
>>> parser.add_argument('input', action='append')
>>> parser.parse_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])
usage: ipython [-h] [-a] [-b] input
ipython: error: unrecognized arguments: filetwo filethree

I’d like this to result in the namespace (a=True, b=True, input=['fileone', 'filetwo', 'filethree']), but cannot see how to do this – if indeed it can. I can’t see anything in the docs or Google which says one way or the other if this is possible, although its quite possible (likely?) I’ve overlooked something. Does anyone have any suggestions?

Asked By: Blair

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Answer 1

You can’t interleave the switches (i.e. -a and -b) with the positional arguments (i.e. fileone, filetwo and filethree) in this way. The switches must appear before or after the positional arguments, not in-between.

Also, in order to have multiple positional arguments, you need to specify the nargs parameter to add_argument. For example:

parser.add_argument('input', nargs='+')

This tells argparse to consume one or more positional arguments and append them to a list. See the argparse documentation for more information. With this line, the code:

parser.parse_args(['-a', '-b', 'fileone', 'filetwo', 'filethree'])

results in:

Namespace(a=True, b=True, input=['fileone', 'filetwo', 'filethree'])

Answered By: srgerg

Answer 2

srgerg was right about the definition of positional arguments. In order to get the result you want, You have to accept them as optional arguments, and modify the resulted namespace according to your need.

You can use a custom action:

class MyAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):

        # Set optional arguments to True or False
        if option_string:
            attr = True if values else False
            setattr(namespace, self.dest, attr)

        # Modify value of "input" in the namespace
        if hasattr(namespace, 'input'):
            current_values = getattr(namespace, 'input')
            try:
                current_values.extend(values)
            except AttributeError:
                current_values = values
            finally:
                setattr(namespace, 'input', current_values)
        else:
            setattr(namespace, 'input', values)

parser = argparse.ArgumentParser()
parser.add_argument('-a', nargs='+', action=MyAction)
parser.add_argument('-b', nargs='+', action=MyAction)
parser.add_argument('input', nargs='+', action=MyAction)

And this is what you get:

>>> parser.parse_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])
Namespace(a=True, b=True, input=['fileone', 'filetwo', 'filethree'])

Or you can modify the resulted namespace like this:

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-a', nargs='+')
>>> parser.add_argument('-b', nargs='+')
>>> parser.add_argument('input', nargs='+')
>>> result = parser.parse_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])

>>> inputs = []
>>> inputs.extend(result.a)
>>> inputs.extend(result.b)
>>> inputs.extend(result.input)

>>> modified = argparse.Namespace(
        a = result.a != [],
        b = result.b != [],
        input = inputs)

And this is what you get:

>>> modified
Namespace(a=True, b=True, input=['filetwo', 'filethree', 'fileone'])

However, both method result in less readable and less maintainable code. Maybe it’s better to change the program logic and do it in a different way.

Answered By: Wang Dingwei

Answer 3

The ‘append’ action makes more sense with an optional:

parser.add_argument('-i', '--input',action='append')
parser.parse_args(['-i','fileone', '-a', '-i','filetwo', '-b', '-i','filethree'])

You can interleave optionals with separate positionals (‘input1 -a input2 -b input3’), but you cannot interleave optionals within one multiitem positional. But you can accomplish this with a two step parse.

import argparse
parser1 = argparse.ArgumentParser()
parser1.add_argument('-a', action='store_true')
parser1.add_argument('-b', action='store_true')
parser2 = argparse.ArgumentParser()
parser2.add_argument('input', nargs='*')

ns, rest = parser1.parse_known_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])
# Namespace(a=True, b=True), ['fileone', 'filetwo', 'filethree']

ns = parser2.parse_args(rest, ns)
# Namespace(a=True, b=True, input=['fileone', 'filetwo', 'filethree'])

http://bugs.python.org/issue14191 is a proposed patch that will do this with single call to:

parser.parse_intermixed_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])

Answered By: hpaulj

Answer 4

It seems to me that hpaulj is on the right track but making things a bit more complicated than necessary. I suspect that Blair is looking for something akin to the behavior of the old optparse module and doesn’t really need the list of input arguments in the inputs field of the args object. He just wants

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-a', action='store_true')
parser.add_argument('-b', action='store_true')
opts, args = parser.parse_known_args(['fileone', '-a', 'filetwo', '-b', 'filethree'])
# Namespace(a=True, b=True), ['fileone', 'filetwo', 'filethree']

In the vernacular of optparse, the “options” are available in opts, and the list of possibly interspersed other “arguments” are in args.

Answered By: Malcolm

Multiple positional arguments with Python and argparse

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