test.support.captured_output is not covered in the online documents:
http://docs.python.org/3.1/library/test.html
http://docs.python.org/dev/py3k/library/test.html

However, it does have a docstring in "C:\Python31\Lib\test\support.py" (see below). The current example for "captured_output" does not work. Looks like someone moved it from "captured_stdout" but did not fully update it. Note the old example still references "captured_stdout". 

# Here's the current code in "support.py":

@contextlib.contextmanager
def captured_output(stream_name):
    """Run the 'with' statement body using a StringIO object in place of a
    specific attribute on the sys module.
    Example use (with 'stream_name=stdout')::

       with captured_stdout() as s:
           print("hello")
       assert s.getvalue() == "hello"
    """
    import io
    orig_stdout = getattr(sys, stream_name)
    setattr(sys, stream_name, io.StringIO())
    try:
        yield getattr(sys, stream_name)
    finally:
        setattr(sys, stream_name, orig_stdout)

def captured_stdout():
    return captured_output("stdout")

# Example for captured_output should now be:

       with captured_output("stdout") as s:
           print("hello")
       assert s.getvalue() == "hello"

# It would be nice to reconcile the online doc versus the docstrings, since it confusing and makes me confused whether captured_stdout is deprecated.