'Returns a specific engine.'

def get_engine(self, app=None, bind=None):

    app = self.get_app(app)
    state = get_state(app)
    with self._engine_lock:
        connector = state.connectors.get(bind)
        if (connector is None):
            connector = self.make_connector(app, bind)
            state.connectors[bind] = connector
        return connector.get_engine()
