> There was a discussion on python-dev about naming conventions in the
> standard library. There was no clear consensus that everything
> non-public should start with an underscore. Several developers thought
> that merely being undocumented or not present in __all__ was
> sufficient (evidently including Guido at the point he implemented
> this).

Underscoring makes it much clearer, though, that people shouldn't rely
on it.
At this point, GCM is only used in copies of the contextlib source code
and our own test_with, so we can still rename it.
(http://www.google.com/codesearch?hl=fr&sa=N&q=GeneratorContextManager )