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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 

This is a duplicate of issue issue 16468, which may get closed with either a bug fix or a doc fix, depending on the outcome of the overall discussion.

This issue has sat idle for six years. Meanwhile, the docs are still incorrect, giving every user wrong information about how the module works. Can we consider just changing the documentation instead of worrying about what the behavior should be or what the rationale is?