argparse - Combining parent parser, subparsers and default values

Question

I wanted to define different subparsers in a script, with both inheriting options from a common parent, but with different defaults. It doesn\'t work as expected, though.

Answer 1

What's happening

The problem here is that parser arguments are objects, and when a parser inherits from it's parents, it adds a reference to the parent's action to it's own list. When you call set_default, it sets the default on this object, which is shared across the subparsers.

You can examine the subparsers to see this:

>>> a1 = [ action for action in subparser1._actions if action.dest=='n' ].pop()
>>> a2 = [ action for action in subparser2._actions if action.dest=='n' ].pop()
>>> a1 is a2 # same object in memory
True
>>> a1.default
20
>>> type(a1)

First solution: Explicitly add this argument to each subparser

You can fix this by adding the argument to each subparser separately rather than adding it to the base class.

subparser1= subparsers.add_parser('a', help='subparser 1', 
                               parents=[base_parser])
subparser1.add_argument('-n', help='number', type=int, default=50)
subparser2= subparsers.add_parser('b', help='subparser 2', 
                               parents=[base_parser])
subparser2.add_argument('-n', help='number', type=int, default=20)
...

Second solution: multiple base classes

If there are many subparsers which share the same default value, and you want to avoid this, you can create different base classes for each default. Since parents is a list of base classes, you can still group the common parts into another base class, and pass the subparser multiple base classes to inherit from. This is probably unnecessarily complicated.

import argparse

# this is the top level parser
parser = argparse.ArgumentParser(description='bla bla')

# this serves as a parent parser
base_parser = argparse.ArgumentParser(add_help=False)
# add common args

# for group with 50 default
base_parser_50 = argparse.ArgumentParser(add_help=False)
base_parser_50.add_argument('-n', help='number', type=int, default=50)

# for group with 50 default
base_parser_20 = argparse.ArgumentParser(add_help=False)
base_parser_20.add_argument('-n', help='number', type=int, default=20)

# subparsers
subparsers = parser.add_subparsers()
subparser1= subparsers.add_parser('a', help='subparser 1', 
                                   parents=[base_parser, base_parser_50])

subparser2 = subparsers.add_parser('b', help='subparser 2',
                                   parents=[base_parser, base_parser_20])

args = parser.parse_args()
print args

First solution with shared args

You can also share a dictionary for the arguments and use unpacking to avoid repeating all the arguments:

import argparse

# this is the top level parser
parser = argparse.ArgumentParser(description='bla bla')

n_args = '-n',
n_kwargs = {'help': 'number', 'type': int}

# subparsers
subparsers = parser.add_subparsers()
subparser1= subparsers.add_parser('a', help='subparser 1')
subparser1.add_argument(*n_args, default=50, **n_kwargs)

subparser2 = subparsers.add_parser('b', help='subparser 2')
subparser2.add_argument(*n_args, default=20, **n_kwargs)

args = parser.parse_args()
print args