Yes, I did miss Victor's dup2() comment.  (It looks like I did not subscribe to this issue from the start and missed early discussion - sorry.)

The simple feature is not very useful for me.  I have to deal with too many cases of misguided code like this:

def write_xyz(output=sys.stdout):
   ...

for which 

with RedirectStdout(...):
    write_xyz()

will not work.

I will create a separate issue once I have a concrete proposal, but with respect to this specific issue, I think it is better to provide a recipe in contextlib documentation for something like this: 

@contextlib.contextmanager
def redirect_stdout(stream):
    old_stdout = sys.stdout
    sys.stdout = stream
    yield
    sys.stdout = old_stdout

With the proposed RedirectStdout, I think many users will want some tweaks and will copy the "from scratch" implementation instead of discovering contextmanager.