Python argparse – Mutually exclusive group with default if no argument is given

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You could have each of your actions update the same variable, supplying stdout as the default value for that variable.

Consider this program:

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument(
    '-s', '--stdout', action='store_const', dest="type", const="s", default="s")
group.add_argument(
    '-c', '--csv', action='store_const', dest="type", const="c")
group.add_argument(
    '-t', '--txt', action='store_const', dest="type", const="t")
args = parser.parse_args()
print args

Your code could look like:

if args.type == 's':
    ofile = sys.stdout
elif args.type == 'c':
    ofile = ...
...

First alternative:

Rather than arbitrarily choose one of the .add_argument()s to specify the default type, you can use parser.set_defaults() to specify the default type. 

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-s', '--stdout', action='store_const', dest="type", const="s")
group.add_argument('-c', '--csv', action='store_const', dest="type", const="c")
group.add_argument('-t', '--txt', action='store_const', dest="type", const="t")
parser.set_defaults(type="s")
args = parser.parse_args()
print args

Second alternative:

Rather than specify the type as an enumerated value, you could store a callable into the type, and then invoke the callable:

import argparse

def do_stdout():
    # do everything that is required to support stdout
    print("stdout!")
    return
def do_csv():
    # do everything that is required to support CSV file
    print("csv!")
    return
def do_text():
    # do everything that is required to support TXT file
    print("text!")
    return

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-s', '--stdout', action='store_const', dest="type", const=do_stdout)
group.add_argument('-c', '--csv', action='store_const', dest="type", const=do_csv)
group.add_argument('-t', '--txt', action='store_const', dest="type", const=do_text)
parser.set_defaults(type=do_stdout)
args = parser.parse_args()
print args
args.type()

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