Any object that supports the in operator can be passed as the choices value, so dict objects, set objects, custom containers, etc. are all supported.  (from  http://docs.python.org/dev/library/argparse.html#choices )

Actual behaviour is contradicted by the docs - it seems the container must additionally be iterable.  When using a custom container, argparse b0rks on trying to iterate through choices.  Using a metavar prevents this, but it still breaks on cases where the element is not in the container.  

More details here:  
http://stackoverflow.com/questions/13833566/python-argparse-choices-from-an-infinite-set