How can I have a default sub-command, or handle the case where no sub-command is given using argparse?

import argparse

a = argparse.ArgumentParser()
b = a.add_subparsers()
b.add_parser('hi')
a.parse_args()

Here I’d like a command to be selected, or the arguments to be handled based only on the next highest level of parser (in this case the top-level parser).

[email protected]:~/src> python3 default_subcommand.py
usage: default_subcommand.py [-h] {hi} ...
default_subcommand.py: error: too few arguments

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')

It seems I’ve stumbled on the solution eventually myself.

If the command is optional, then this makes the command an option. In my original parser configuration, I had a package command that could take a range of possible steps, or it would perform all steps if none was given. This makes the step a choice:

parser = argparse.ArgumentParser()

command_parser = subparsers.add_parser('command')
command_parser.add_argument('--step', choices=['prepare', 'configure', 'compile', 'stage', 'package'])

...other command parsers

parsed_args = parser.parse_args()

if parsed_args.step is None:
    do all the steps...

Here’s a nicer way of adding a set_default_subparser method:

class DefaultSubcommandArgParse(argparse.ArgumentParser):
    __default_subparser = None

    def set_default_subparser(self, name):
        self.__default_subparser = name

    def _parse_known_args(self, arg_strings, *args, **kwargs):
        in_args = set(arg_strings)
        d_sp = self.__default_subparser
        if d_sp is not None and not {'-h', '--help'}.intersection(in_args):
            for x in self._subparsers._actions:
                subparser_found = (
                    isinstance(x, argparse._SubParsersAction) and
                    in_args.intersection(x._name_parser_map.keys())
                )
                if subparser_found:
                    break
            else:
                # insert default in first position, this implies no
                # global options without a sub_parsers specified
                arg_strings = [d_sp] + arg_strings
        return super(DefaultSubcommandArgParse, self)._parse_known_args(
            arg_strings, *args, **kwargs
        )

Maybe what you’re looking for is the dest argument of add_subparsers:

(Warning: works in Python 3.4, but not in 2.7)

import argparse
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest="cmd")
parser_hi = subparsers.add_parser('hi')
parser.parse_args([]) # Namespace(cmd=None)

Now you can just use the value of cmd:

if cmd in [None, 'hi']:
    print('command "hi"')

You can duplicate the default action of a specific subparser on the main parser, effectively making it the default.

import argparse
p = argparse.ArgumentParser()
sp = p.add_subparsers()

a = sp.add_parser('a')
a.set_defaults(func=do_a)

b = sp.add_parser('b')
b.set_defaults(func=do_b)

p.set_defaults(func=do_b)
args = p.parse_args()

if args.func:
    args.func()
else:
    parser.print_help()

Does not work with add_subparsers(required=True), which is why the if args.func is down there.

In my case I found it easiest to explicitly provide the subcommand name to parse_args() when argv was empty.

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(help='commands')

runParser = subparsers.add_parser('run', help='[DEFAULT ACTION]')

altParser = subparsers.add_parser('alt', help='Alternate command')
altParser.add_argument('alt_val', type=str, help='Value required for alt command.')

# Here's my shortcut: If `argv` only contains the script name,
# manually inject our "default" command.
args = parser.parse_args(['run'] if len(sys.argv) == 1 else None)
print args

Example runs:

$ ./test.py 
Namespace()

$ ./test.py alt blah
Namespace(alt_val="blah")

$ ./test.py blah
usage: test.py [-h] {run,alt} ...
test.py: error: invalid choice: 'blah' (choose from 'run', 'alt')

In python 2.7, you can override the error behaviour in a subclass (a shame there isn’t a nicer way to differentiate the error):

import argparse

class ExceptionArgParser(argparse.ArgumentParser):

    def error(self, message):
        if "invalid choice" in message:
            # throw exception (of your choice) to catch
            raise RuntimeError(message)
        else:
            # restore normal behaviour
            super(ExceptionArgParser, self).error(message)


parser = ExceptionArgParser()
subparsers = parser.add_subparsers(title="Modes", dest="mode")

default_parser = subparsers.add_parser('default')
default_parser.add_argument('a', nargs="+")

other_parser = subparsers.add_parser('other')
other_parser.add_argument('b', nargs="+")

try:
    args = parser.parse_args()
except RuntimeError:
    args = default_parser.parse_args()
    # force the mode into namespace
    setattr(args, 'mode', 'default') 

print args

You can add an argument with a default value that will be used when nothing is set I believe.

See this: http://docs.python.org/dev/library/argparse.html#default

Edit:

Sorry, I read your question a bit fast.

I do not think you would have a direct way of doing what you want via argparse. But you could check the length of sys.argv and if its length is 1 (only script name) then you could manually pass the default parameters for parsing, doing something like this:

import argparse

a = argparse.ArgumentParser()
b = a.add_subparsers()
b.add_parser('hi')

if len(sys.argv) == 1:
   a.parse_args(['hi'])
else:
   a.parse_args()

I think that should do what you want, but I agree it would be nice to have this out of the box.

For later reference:

...
b = a.add_subparsers(dest="cmd")
b.set_defaults(cmd='hey')  # <-- this makes hey as default

b.add_parser('hi')

so, these two will be same:

  • python main.py hey
  • python main.py