Call function based on argparse

Since it seems like you want to run one, and only one, function depending on the arguments given, I would suggest you use a mandatory positional argument ./prog command, instead of optional arguments (./prog --command1 or ./prog --command2).

so, something like this should do it:

FUNCTION_MAP = {'top20' : my_top20_func,
                'listapps' : my_listapps_func }

parser.add_argument('command', choices=FUNCTION_MAP.keys())

args = parser.parse_args()

func = FUNCTION_MAP[args.command]
func()

There are lots of ways of skinning this cat. Here's one using action='store_const' (inspired by the documented subparser example):

p=argparse.ArgumentParser()
p.add_argument('--cmd1', action='store_const', const=lambda:'cmd1', dest='cmd')
p.add_argument('--cmd2', action='store_const', const=lambda:'cmd2', dest='cmd')

args = p.parse_args(['--cmd1'])
# Out[21]: Namespace(cmd=<function <lambda> at 0x9abf994>)

p.parse_args(['--cmd2']).cmd()
# Out[19]: 'cmd2'
p.parse_args(['--cmd1']).cmd()
# Out[20]: 'cmd1'

With a shared dest, each action puts its function (const) in the same Namespace attribute. The function is invoked by args.cmd().

And as in the documented subparsers example, those functions could be written so as to use other values from Namespace.

args = parse_args()
args.cmd(args)

For sake of comparison, here's the equivalent subparsers case:

p = argparse.ArgumentParser()
sp = p.add_subparsers(dest='cmdstr')
sp1 = sp.add_parser('cmd1')
sp1.set_defaults(cmd=lambda:'cmd1')
sp2 = sp.add_parser('cmd2')
sp2.set_defaults(cmd=lambda:'cmd2')

p.parse_args(['cmd1']).cmd()
# Out[25]: 'cmd1'

As illustrated in the documentation, subparsers lets you define different parameter arguments for each of the commands.

And of course all of these add argument or parser statements could be created in a loop over some list or dictionary that pairs a key with a function.

Another important consideration - what kind of usage and help do you want? The different approaches generate very different help messages.

At least from what you have described, --showtop20 and --listapps sound more like sub-commands than options. Assuming this is the case, we can use subparsers to achieve your desired result. Here is a proof of concept:

import argparse
import sys

def showtop20():
    print('running showtop20')

def listapps():
    print('running listapps')

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()

# Create a showtop20 subcommand    
parser_showtop20 = subparsers.add_parser('showtop20', help='list top 20 by app')
parser_showtop20.set_defaults(func=showtop20)

# Create a listapps subcommand       
parser_listapps = subparsers.add_parser('listapps', help='list all available apps')
parser_listapps.set_defaults(func=listapps)

# Print usage message if no args are supplied.

# NOTE: Python 2 will error 'too few arguments' if no subcommand is supplied.
#       No such error occurs in Python 3, which makes it feasible to check
#       whether a subcommand was provided (displaying a help message if not).
#       argparse internals vary significantly over the major versions, so it's
#       much easier to just override the args passed to it.

if len(sys.argv) <= 1:
    sys.argv.append('--help')

options = parser.parse_args()

# Run the appropriate function (in this case showtop20 or listapps)
options.func()

# If you add command-line options, consider passing them to the function,
# e.g. `options.func(options)`