def contextmanager(func):
    @wraps(func)
    def helper(*args, **kwds):
        return GeneratorContextManager(func(*args, **kwds))
    return helper

class GeneratorContextManager(object):
    """Helper for @contextmanager decorator."""

    def __init__(self, gen):
        self.gen = gen

    def __enter__(self):
        return self.gen.next()

    def __exit__(self, type, value, traceback):
        if type is None:
            try:
                self.gen.next()
            except StopIteration:
                return
            else:
                raise RuntimeError("generator didn't stop")

@contextmanager
def tag(name):
    print "<%s>" % name
    yield
    print "</%s>" % name

>>> with tag("h1"):
...    print "foo"
...
<h1>
foo
</h1>