One thing that Jack's confusion above does suggest to me is that we should mention in the *ContextDecorator* documentation that it is automatically applied to the context managers created when you use @contextmanager. A lot of people familiar with contextmanager are just going to read the docs for the new toy, so may miss the fact that we have added __call__ support to GeneratorContextManager.

As far as use cases go, this change is just syntactic sugar for any construct of the following form:

  def f():
    with cm():
      # Do stuff

ContextDecorator lets you instead write:

  @cm
  def f():
    # Do stuff

It makes it clear that the CM applies to the whole function, rather than just a piece of it (and saving an indentation level is nice, too).