Default sub-command, or handling no sub-command with argparse

On Python 3.2 (and 2.7) you will get that error, but not on 3.3 and 3.4 (no response). Therefore on 3.3/3.4 you could test for parsed_args to be an empty Namespace.

A more general solution is to add a method set_default_subparser() (taken from the ruamel.std.argparse package) and call that method just before parse_args():

import argparse
import sys

def set_default_subparser(self, name, args=None, positional_args=0):
    """default subparser selection. Call after setup, just before parse_args()
    name: is the name of the subparser to call by default
    args: if set is the argument list handed to parse_args()

    , tested with 2.7, 3.2, 3.3, 3.4
    it works with 2.6 assuming argparse is installed
    """
    subparser_found = False
    for arg in sys.argv[1:]:
        if arg in ['-h', '--help']:  # global help if no subparser
            break
    else:
        for x in self._subparsers._actions:
            if not isinstance(x, argparse._SubParsersAction):
                continue
            for sp_name in x._name_parser_map.keys():
                if sp_name in sys.argv[1:]:
                    subparser_found = True
        if not subparser_found:
            # insert default in last position before global positional
            # arguments, this implies no global options are specified after
            # first positional argument
            if args is None:
                sys.argv.insert(len(sys.argv) - positional_args, name)
            else:
                args.insert(len(args) - positional_args, name)

argparse.ArgumentParser.set_default_subparser = set_default_subparser

def do_hi():
    print('inside hi')

a = argparse.ArgumentParser()
b = a.add_subparsers()
sp = b.add_parser('hi')
sp.set_defaults(func=do_hi)

a.set_default_subparser('hi')
parsed_args = a.parse_args()

if hasattr(parsed_args, 'func'):
    parsed_args.func()

This will work with 2.6 (if argparse is installed from PyPI), 2.7, 3.2, 3.3, 3.4. And allows you to do both

python3 default_subcommand.py

and

python3 default_subcommand.py hi

with the same effect.

Allowing to chose a new subparser for default, instead of one of the existing ones.

The first version of the code allows setting one of the previously-defined subparsers as a default one. The following modification allows adding a new default subparser, which could then be used to specifically process the case when no subparser was selected by user (different lines marked in the code)

def set_default_subparser(self, name, args=None, positional_args=0):
    """default subparser selection. Call after setup, just before parse_args()
    name: is the name of the subparser to call by default
    args: if set is the argument list handed to parse_args()

    , tested with 2.7, 3.2, 3.3, 3.4
    it works with 2.6 assuming argparse is installed
    """
    subparser_found = False
    existing_default = False # check if default parser previously defined
    for arg in sys.argv[1:]:
        if arg in ['-h', '--help']:  # global help if no subparser
            break
    else:
        for x in self._subparsers._actions:
            if not isinstance(x, argparse._SubParsersAction):
                continue
            for sp_name in x._name_parser_map.keys():
                if sp_name in sys.argv[1:]:
                    subparser_found = True
                if sp_name == name: # check existance of default parser
                    existing_default = True
        if not subparser_found:
            # If the default subparser is not among the existing ones,
            # create a new parser.
            # As this is called just before 'parse_args', the default
            # parser created here will not pollute the help output.

            if not existing_default:
                for x in self._subparsers._actions:
                    if not isinstance(x, argparse._SubParsersAction):
                        continue
                    x.add_parser(name)
                    break # this works OK, but should I check further?

            # insert default in last position before global positional
            # arguments, this implies no global options are specified after
            # first positional argument
            if args is None:
                sys.argv.insert(len(sys.argv) - positional_args, name)
            else:
                args.insert(len(args) - positional_args, name)

argparse.ArgumentParser.set_default_subparser = set_default_subparser

a = argparse.ArgumentParser()
b = a.add_subparsers(dest="cmd")
sp = b.add_parser('hi')
sp2 = b.add_parser('hai')

a.set_default_subparser('hey')
parsed_args = a.parse_args()

print(parsed_args)

The “default” option will still not show up in the help:

python test_parser.py -h
usage: test_parser.py [-h] {hi,hai} ...

positional arguments:
  {hi,hai}

optional arguments:
  -h, --help  show this help message and exit

However, it is now possible to differentiate between and separately handle calling one of the provided subparsers, and calling the default subparser when no argument was provided:

$ python test_parser.py hi
Namespace(cmd='hi')
$ python test_parser.py 
Namespace(cmd='hey')