I answered a similar question recently on Stackoverflow when the user wanted to use `type=hex`.

http://stackoverflow.com/questions/37006387/python-argparse-hex-error


In another recent bug/issue the poster want a `enum` type.  It's not hard to define a function, or factory class, that handles mappings like this, but it isn't as simple or intuitive as some would like.

There is a registries mechanism, which could allow the user to use `type='bool'`, where 'bool' is the name of a function that that converts some set of strings to True/False.  But people are used to providing strings for the `action`, they aren't used to do so for the `type`.

http://stackoverflow.com/questions/15008758/parsing-boolean-values-with-argparse