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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, req):
 """Determine if ``req`` is in this instances cache. Determine if there are cache hits for the request in this aggregator instance. Not in the cache If req is not in the cache, it returns ``None`` to indicate that the caller should send the request. Cache Hit; response has errors When a cached CheckResponse has errors, it's assumed that ``req`` would fail as well, so the cached CheckResponse is returned. However, the first CheckRequest after the flush interval has elapsed should be sent to the server to refresh the CheckResponse, though until it's received, subsequent CheckRequests should fail with the cached CheckResponse. Cache behaviour - response passed If the cached CheckResponse has no errors, it's assumed that ``req`` will succeed as well, so the CheckResponse is returned, with the quota info updated to the same as requested. The requested tokens are aggregated until flushed. Args: req (``ServicecontrolServicesCheckRequest``):
 to be sent to the service control service Raises: ValueError: if the ``req`` service_name is not the same as this instances Returns: ``CheckResponse``: if an applicable response is cached by this instance is available for use or None, if there is no applicable response """

        if self._cache is None:
            return None  # no cache, send request now
        if not isinstance(req, sc_messages.ServicecontrolServicesCheckRequest):
            raise ValueError(u'Invalid request')
        if req.serviceName != self.service_name:
            _logger.error(u'bad check(): service_name %s does not match ours %s',
                          req.serviceName, self.service_name)
            raise ValueError(u'Service name mismatch')
        check_request = req.checkRequest
        if check_request is None:
            _logger.error(u'bad check(): no check_request in %s', req)
            raise ValueError(u'Expected operation not set')
        op = check_request.operation
        if op is None:
            _logger.error(u'bad check(): no operation in %s', req)
            raise ValueError(u'Expected operation not set')
        if op.importance != sc_messages.Operation.ImportanceValueValuesEnum.LOW:
            return None  # op is important, send request now

        signature = sign(check_request)
        with self._cache as cache:
            _logger.debug(u'checking the cache for %r\n%s', signature, cache)
            item = cache.get(signature)
            if item is None:
                return None  # signal to caller to send req
            else:
                return self._handle_cached_response(req, item)