Argparse default trick

At One Identity - Quest we have a repetitive job to create small helper scripts for our internal services. Most of them are simple xmlrpc client which expose only a couple of methods and the methods have zero or one parameter. This minimalist requirements can be implemented with standard python libraries.

argparse subparsers

The add_subparsers method of ArgumentParser can be used to select the xmlrpc method:

import argparse
import xmlrpc.client

parser = argparse.ArgumentParser()
sub_parsers = parser.add_subparsers(dest="command", required=True)

sub_parsers.add_parser("list")

get_parser = sub_parsers.add_parser("get")
get_parser.add_argument("id")

client = xmlrpc.client.ServerProxy("http://127.0.0.1:1234")
args = parser.parse_args()

if args.command == "list":
    client.list()
elif args.command == "get":
    client.get(args.id)

and argparse will generate the usual interactive help for our client:

usage: client.py [-h] {list,get} ...

positional arguments:
  {list,get}

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

usage: client.py get [-h] id

positional arguments:
  id

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

set_defaults

This is nice, but it would better if the function call and the argument definition would be closer. The set_defaults method can help to register our xmlrpc call into the parsed args:

import argparse
import xmlrpc.client

client = xmlrpc.client.ServerProxy("http://127.0.0.1:1234")

parser = argparse.ArgumentParser()
sub_parsers = parser.add_subparsers(dest="command", required=True)

list_parser = sub_parsers.add_parser("list")
list_parser.set_defaults(action=lambda _: client.list())

get_parser = sub_parsers.add_parser("get")
get_parser.add_argument("id")
get_parser.set_defaults(action=lambda args: client.get(args.id))

args = parser.parse_args()
args.action(args)

argparse_action

It's more compact but not so pythonic. The code would be more readable if the argument definition would be created from the function signature not the other way around. The inspect.signature can help us to create a helper module (argparse_action) for this:

import inspect

def add_action(sub_parsers, func):
    parser = sub_parsers.add_parser(func.__name__)
    sig = inspect.signature(func)

    for name in sig.parameters:
        parser.add_argument(name)

    def action(cli_args):
        func_args = [getattr(cli_args, name) for name in sig.parameters]
        return func(*func_args)

    parser.set_defaults(action=action)

import argparse
import xmlrpc.client

from argparse_action import add_action


def get(id):
    client = _connect()
    return client.get(id)

def list():
    client = _connect()
    return client.list()

def _connect():
    return xmlrpc.client.ServerProxy("http://127.0.0.1:1234")


parser = argparse.ArgumentParser()
sub_parsers = parser.add_subparsers(dest="command", required=True)
add_action(sub_parsers, list)
add_action(sub_parsers, get)

args = parser.parse_args()
args.action(args)

action.add decorator

It is more readable but we cannot see easily which function is exposed to CLI. A decoration could help:

import inspect


class Action:
    def __init__(self, pasrer):
        self._parsers = parser.add_subparsers(dest="command", required=True)

    def add(self, func):
        parser = self._parsers.add_parser(func.__name__)
        return add_action(parser, func)

def add_action(parser, func):
    sig = inspect.signature(func)

    for name in sig.parameters:
        parser.add_argument(name)

    def action(cli_args):
        func_args = [getattr(cli_args, name) for name in sig.parameters]
        return func(*func_args)

    parser.set_defaults(action=action)
    return action

import argparse
import xmlrpc.client

from argparse_action import add_action

parser = argparse.ArgumentParser()
action = Action(parser)


@action.add()
def get(id):
    client = _connect()
    return client.get(id)

@action.add()
def list():
    client = _connect()
    return client.list()

def _connect():
    return xmlrpc.client.ServerProxy("http://127.0.0.1:1234")

args = parser.parse_args()
args.action(args)

The argparse_action is published to pypi so you can try the its other features.