To simplify the discussion and for issue resolution purposes, I propose that the discussion about "large" choices containers be divided into separate discussions for (1) changes that should be applied to all maintenance releases (i.e. bug fix changes), and (2) changes that should be applied only to the in-development branch (i.e. enhancements).

I propose that the current issue be used for the former.  3.4-only enhancements can be dealt with as part of a separate issue.

I also created issue 16468 for the bug that Terry observed above that ArgumentParser does not in general support "choices" values that support the "in" operator.  That issue exists and can be resolved independent of whether the choices collection is large.