How to handle CLI subcommands with argparse

Question

I need to implement a command line interface in which the program accepts subcommands.

For example, if the program is called “foo”, the CLI would look like

foo cmd1 <cmd1-options>
foo cmd2
foo cmd3 <cmd3-options>

cmd1 and cmd3 must be used with at least one of their options and the three cmd* arguments are always exclusive.

I am trying to use subparsers in argparse, but with no success for the moment. The problem is with cmd2, that has no arguments:

if I try to add the subparser entry with no arguments, the namespace returned by parse_args will not contain any information telling me that this option was selected (see the example below). if I try to add cmd2 as an argument to the parser (not the subparser), then argparse will expect that the cmd2 argument will be followed by any of the subparsers arguments.

Is there a simple way to achieve this with argparse? The use case should be quite common…

Here follows what I have attempted so far that is closer to what I need:

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(help='Functions')
parser_1 = subparsers.add_parser('cmd1', help='...')
parser_1.add_argument('cmd1_option1', type=str, help='...')

parser_2 = subparsers.add_parser(cmd2, help='...')

parser_3 = subparsers.add_parser('cmd3', help='...')
parser_3.add_argument('cmd3_options', type=int, help='...')

args = parser.parse_args()

Pontic

Answer 1

First of all subparsers are never inserted in the namespace. In the example you posted if you try to run the script as:

$python3 test_args.py cmd1 1
Namespace(cmd1_option1='1')

where test_args.py contain the code you provided (with the import argparse at the beginning and print(args) at the end).

Note that there is no mention to cmd1 only to its argument. This is by design.

As pointed out in the comments you can add that information passing the dest argument to the add_subparsers call.

The usual way to handle these circumstances is to use the set_defaults method of the subparsers:

import argparse
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(help='Functions')
parser_1 = subparsers.add_parser('cmd1', help='...')
parser_1.add_argument('cmd1_option1', type=str, help='...')
parser_1.set_defaults(parser1=True)

parser_2 = subparsers.add_parser('cmd2', help='...')
parser_2.set_defaults(parser2=True)

parser_3 = subparsers.add_parser('cmd3', help='...')
parser_3.add_argument('cmd3_options', type=int, help='...')
parser_3.set_defaults(parser_3=True)

args = parser.parse_args()
print(args)

Which results in:

$python3 test_args.py cmd1 1
Namespace(cmd1_option1='1', parser1=True)
$python3 test_args.py cmd2
Namespace(parser2=True)

In general different subparser will, most of the time, handle the arguments in completely different ways. The usual pattern is to have different functions to run the different commands and use set_defaults to set a func attribute. When you parse the arguments you simply call that callable:

subparsers = parser.add_subparsers()
parser_1 = subparsers.add_parser(...)
parser_1.set_defaults(func=do_command_one)

parser_k = subparsers.add_parser(...)
parser_k.set_defaults(func=do_command_k)

args = parser.parse_args()
if args.func:
    args.func(args)

Evesham

I take issue with the 'never inserted'. Simply give the 'add_subparsers' a dest. – Sukey

@Sukey You are right. However I, personally, never used that. As I said it's much more command to have different handling functions for different subparsers so using set_defaults is a better solution. Only for the most simple situations you want to use the dest parameter. – Evesham

Answer 2

The subparser identity can be added to the main Namespace if the add_subparsers command is given a dest.

From the documentation:

However, if it is necessary to check the name of the subparser that was invoked, the dest keyword argument to the add_subparsers() call will work:

>>> parser = argparse.ArgumentParser()
>>> subparsers = parser.add_subparsers(dest='subparser_name')
>>> subparser1 = subparsers.add_parser('1')
>>> subparser1.add_argument('-x')
>>> subparser2 = subparsers.add_parser('2')
>>> subparser2.add_argument('y')
>>> parser.parse_args(['2', 'frobble'])
Namespace(subparser_name='2', y='frobble')

By default the dest is argparse.SUPPRESS, which keeps subparsers from adding the name to the namespace.

Sukey