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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def check(self, check_req):
 """Process a check_request. The req is first passed to the check_aggregator. If there is a valid cached response, that is returned, otherwise a response is obtained from the transport. Args: check_req (``ServicecontrolServicesCheckRequest``):
 to be sent to the service control service Returns: ``CheckResponse``: either the cached response if one is applicable or a response from making a transport request, or None if if the request to the transport fails """


        self.start()
        res = self._check_aggregator.check(check_req)
        if res:
            _logger.debug(u'using cached check response for %s: %s',
                          check_request, res)
            return res

        # Application code should not fail because check request's don't
        # complete, They should fail open, so here simply log the error and
        # return None to indicate that no response was obtained
        try:
            transport = self._create_transport()
            resp = transport.services.Check(check_req)
            self._check_aggregator.add_response(check_req, resp)
            return resp
        except exceptions.Error:  # only sink apitools errors
            _logger.error(u'direct send of check request failed %s',
                          check_request, exc_info=True)
            return None