Python argparse subparser dest parameter doesn't work with a parent

Question

I'm writing a utility that will have multiple modules, and which module gets run is determined by an argument. Each module has it's own arguments, but all modules will share 4 standard arguments. To get this to work I just set the 'parent' param when creating the subparsers, but the problem is I also need to be able to determine which module was called on the command line. It looks like the 'dest' param is the way to do this, but for some reason having both 'parent' and 'dest' set at the same time does not work.

import argparse

parser = argparse.ArgumentParser() # main parser
parser.addArgument("--foo", action='store_true')

subparsers = parser.add_subparsers(dest='cmd')
# without 'parents=[parser]' it properly stores 'bar' in cmd
# however '--foo' MUST be before 'bar'
bar = subparsers.add_parser("bar", parents=[parser], add_help=False)
bar.add_argument("--test", action='store_true')

# should be able to have '--foo' before OR after 'bar'
parser.parse_args(['--foo', 'bar', '--test'])

In this code, the add_subparsers call sets the dest to 'cmd.' Then, I could parse the arguments and call args.cmd to get the name of the module called (in this case, bar). However when parents is set the value of cmd is always None. Currently my workaround is to just have an empty main parser and simply copy-paste the 4 standard args to every subparser, which works but is not exactly desirable.

My question: Is there another way to determine which module was called? Why does this even happen?

Answer 1

Thanks to the information provided by @hpaulj in the comment to the OP, I managed to get this working.

Basically, you need your main parser and a parent parser. You will then set the parents attribute of your subparsers to be the parent parser. Based on the example you gave, the following should be a working example:

import argparse

# Create parsers
parser = argparse.ArgumentParser() 
parent_parser = argparse.ArgumentParser()

# Add arguments to parent parser
parent_parser.add_argument("--foo", action='store_true')

# Create subparser
subparsers = parser.add_subparsers(dest='cmd')

# Add to the subparser
bar = subparsers.add_parser("bar", parents=[parent_parser], add_help=False)
bar.add_argument("--test", action='store_true')
baz = subparsers.add_parser("baz", parents=[parent_parser], add_help=False)
baz.add_argument("--baz-test", action="store_true")

# should be able to have '--foo' before OR after 'bar'
print(parser.parse_args(['bar', '--test']))
print(parser.parse_args(["baz", "--baz-test"]))

This outputs the following:

Namespace(cmd='bar', foo=False, test=True)
Namespace(baz_test=True, cmd='baz', foo=False)

You should then be able to do things like this:

args = parser.parse_args()
if args.cmd == "bar":
    print("bar was specified")
elif args.cmd == "baz":
    print("baz was specified")

It might not be the perfect solution, but it should work.

(Tested using Python 3.5.2)