controller.py

def bracket(first, last, between):
    """
    Invoke an action between two other actions.

    :param first: A no-argument function that may return a Deferred.  It is
        called first.

    :param last: A no-argument function that may return a Deferred.  It is
        called last.

    :param between: A no-argument function that may return a Deferred.  It is
        called after ``first`` is done and completes before ``last`` is called.

    :return Deferred: A ``Deferred`` which fires with the result of
        ``between``.
    """
    yield first()
    try:
        result = yield between()
    except GeneratorExit:
        raise
    except:
        info = exc_info()
        yield last()
        raise info[0], info[1], info[2]
    else:
        yield last()
        returnValue(result)