[Python-ideas] Suggestion: Collection type argument for argparse where nargs != None

[David Mayo]

A friend of mine (@bcjbcjbcj on twitter) came up with an idea for an
argparse improvement that I'd like to propose for inclusion.

Currently, argparse with nargs=<anything but None> collects arguments into
a list (or a list of lists in the case of action="append"). I would like to
propose adding a "collection type" argument to the store and append actions
and to add_argument, consisting of a callable that would be applied to the
list of type-converted args before adding them to the Namespace. This would
allow for alternate constructors (e.g. set), for modifying the list (e.g.
with sorted), or to do checking of properties expected across all
components of the argument at parse time.

I've worked up a set of examples in this gist:
https://gist.github.com/pobocks/bff0bea494f2b7ec7eba1e8ae281b888

And a rough implementation here:
https://github.com/python/cpython/compare/master...pobocks:argparse_colltype

I think this would be genuinely useful, and would require very little
change to argparse, which should be backwards compatible provided that the
default for the collection type is list, or None with list specified if
None.

Thank you all for your time in considering this,

- Dave Mayo
@pobocks on twitter, github, various others
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