paul j3 <ajipa...@gmail.com> added the comment:

I've explored something similar in 

https://bugs.python.org/issue11588
Add "necessarily inclusive" groups to argparse

There is a local variable in parser._parse_known_args

    seen_non_default_actions

that's a set of the actions that have been seen.  It is used for testing for 
required actions, and for mutually_exclusive groups.  But making it available 
to users without altering user API code is awkward.

My latest idea was to add it as an attribute to the parser, or (conditionally) 
as attribute of the namespace

https://bugs.python.org/issue11588#msg265734

I've also thought about tweaking the interface between

parser._parse_known_args
parser.parse_known_args

to do of more of the error checking in the caller, and give the user more 
opportunity to do their checks. This variable would be part of 
_parse_known_args output.

 
Usually though when testing like this comes up on SO, I suggest leaving the 
defaults as None, and then just using a 

     if args.foobar is None:
          # not seen

Defaults are written to the namespace at the start of parsing, and seen 
arguments overwrite those values (with an added type 'eval' step of remaining 
defaults at the end).


Keep in mind, though, that the use of subparsers could complicate any of these 
tweaks.

In reading my posts on https://bugs.python.org/issue26394, I remembered the 
IPython uses argparse (subclassed) with config.  I believe it uses config 
inputs (default and user) to define the arguments for the parser.


So unless someone comes up with a really clever idea, this is bigger request 
than it first impressions suggest.

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