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Related: http://bugs.python.org/issue9253
I came across issue9253 in trying to implement a default action for a subparser namespace. In the absence of a 'default' option, I thought that it may be possible by adding an 'action' to 'add_subparsers'. Per the documentation this should be possible:
https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser.add_subparsers
[QUOTE]
action - the basic type of action to be taken when this argument is 
encountered at the command line
[/QUOTE]
That said, custom actions on 'add_subparsers' doesn't appear to work at all:
 import argparse
 class CustomAction(argparse.Action):
 def __call__(self, parser, namespace, values, option_string=None):
 print('Inside CustomAction')
 setattr(namespace, self.dest, values)
 root_parser = argparse.ArgumentParser(prog='myapp')
 sub_parser = root_parser.add_subparsers(dest='commands', action=CustomAction)
 args = root_parser.parse_args()
Produces:
$ python argtest.py --help
Traceback (most recent call last):
 File "argtest.py", line 46, in <module>
 sub_parser = root_parser.add_subparsers(dest='commands', action=CustomAction)
 File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/argparse.py", line 1661, in add_subparsers
 action = parsers_class(option_strings=[], **kwargs)
TypeError: __init__() got an unexpected keyword argument 'prog'
Erroneous documentation maybe? Tested the same on Python 2.7 and 3.3.