text: import operator
  from collections import defaultdict
  
  from cliquet import utils
  from cliquet.storage import (
      StorageBase, exceptions, Filter,
      DEFAULT_ID_FIELD, DEFAULT_MODIFIED_FIELD, DEFAULT_DELETED_FIELD)
  from cliquet.utils import COMPARISON
  
  
  def tree():
      return defaultdict(tree)
  
  
  class MemoryBasedStorage(StorageBase):
      """Abstract storage class, providing basic operations and
      methods for in-memory implementations of sorting and filtering.
      """
      def __init__(self, *args, **kwargs):
          pass
  
      def initialize_schema(self):
          # Nothing to do.
          pass
  
      def delete_all(self, collection_id, parent_id, filters=None,
                     id_field=DEFAULT_ID_FIELD, with_deleted=True,
                     modified_field=DEFAULT_MODIFIED_FIELD,
                     deleted_field=DEFAULT_DELETED_FIELD,
                     auth=None):
          records, count = self.get_all(collection_id, parent_id,
                                        filters=filters,
                                        id_field=id_field,
                                        modified_field=modified_field,
                                        deleted_field=deleted_field)
          deleted = [self.delete(collection_id, parent_id, r[id_field],
                                 id_field=id_field, with_deleted=with_deleted,
                                 modified_field=modified_field,
                                 deleted_field=deleted_field)
                     for r in records]
          return deleted
  
      def strip_deleted_record(self, resource, parent_id, record,
                               id_field=DEFAULT_ID_FIELD,
                               modified_field=DEFAULT_MODIFIED_FIELD,
                               deleted_field=DEFAULT_DELETED_FIELD):
          """Strip the record of all its fields expect id and timestamp,
          and set the deletion field value (e.g deleted=True)
          """
          deleted = {}
          deleted[id_field] = record[id_field]
          deleted[modified_field] = record[modified_field]
  
          deleted[deleted_field] = True
          return deleted
  
      def set_record_timestamp(self, collection_id, parent_id, record,
                               modified_field=DEFAULT_MODIFIED_FIELD,
                               last_modified=None):
          timestamp = self._bump_timestamp(collection_id, parent_id, record,
                                           modified_field,
                                           last_modified=last_modified)
          record[modified_field] = timestamp
          return record
  
      def check_unicity(self, collection_id, parent_id, record,
                        unique_fields, id_field, for_creation=False):
          """Check that the specified record does not violates unicity
          constraints defined in the resource's mapping options.
          """
          if for_creation and id_field in record:
              # If id is provided by client, check that no record conflicts.
              unique_fields = (unique_fields or tuple()) + (id_field,)
  
          if not unique_fields:
              return
  
          unicity_rules = get_unicity_rules(collection_id, parent_id, record,
                                            unique_fields=unique_fields,
                                            id_field=id_field,
                                            for_creation=for_creation)
          for filters in unicity_rules:
              existing, count = self.get_all(collection_id, parent_id,
                                             filters=filters,
                                             id_field=id_field)
              if count > 0:
                  field = filters[0].field
                  raise exceptions.UnicityError(field, existing[0])
  
      def apply_filters(self, records, filters):
          """Filter the specified records, using basic iteration.
          """
          operators = {
              COMPARISON.LT: operator.lt,
              COMPARISON.MAX: operator.le,
              COMPARISON.EQ: operator.eq,
              COMPARISON.NOT: operator.ne,
              COMPARISON.MIN: operator.ge,
              COMPARISON.GT: operator.gt,
              COMPARISON.IN: operator.contains,
              COMPARISON.EXCLUDE: lambda x, y: not operator.contains(x, y),
          }
  
          for record in records:
              matches = True
              for f in filters:
                  left = record.get(f.field)
                  right = f.value
                  if f.operator in (COMPARISON.IN, COMPARISON.EXCLUDE):
                      right = left
                      left = f.value
                  matches = matches and operators[f.operator](left, right)
              if matches:
                  yield record
  
      def apply_sorting(self, records, sorting):
          """Sort the specified records, using cumulative python sorting.
          """
          return apply_sorting(records, sorting)
  
      def extract_record_set(self, collection_id, records,
                             filters, sorting, id_field, deleted_field,
                             pagination_rules=None, limit=None):
          """Take the list of records and handle filtering, sorting and
          pagination.
  
          """
          filtered = list(self.apply_filters(records, filters or []))
          total_records = len(filtered)
  
          paginated = {}
          for rule in pagination_rules or []:
              values = list(self.apply_filters(filtered, rule))
              paginated.update(dict(((x[id_field], x) for x in values)))
  
          if paginated:
              paginated = paginated.values()
          else:
              paginated = filtered
  
          sorted_ = self.apply_sorting(paginated, sorting or [])
  
          filtered_deleted = len([r for r in sorted_
                                  if r.get(deleted_field) is True])
  
          if limit:
              sorted_ = list(sorted_)[:limit]
  
          return sorted_, total_records - filtered_deleted
  
  
  class Storage(MemoryBasedStorage):
      """Storage backend implementation in memory.
  
      Useful for development or testing purposes, but records are lost after
      each server restart.
  
      Enable in configuration::
  
          cliquet.storage_backend = cliquet.storage.memory
      """
      def __init__(self, *args, **kwargs):
          super(Storage, self).__init__(*args, **kwargs)
          self.flush()
  
      def flush(self, auth=None):
          self._store = tree()
          self._cemetery = tree()
          self._timestamps = defaultdict(dict)
  
      def collection_timestamp(self, collection_id, parent_id, auth=None):
          ts = self._timestamps[collection_id].get(parent_id)
          if ts is not None:
              return ts
          return self._bump_timestamp(collection_id, parent_id)
  
      def _bump_timestamp(self, collection_id, parent_id, record=None,
                          modified_field=None, last_modified=None):
          """Timestamp are base on current millisecond.
  
          .. note ::
  
              Here it is assumed that if requests from the same user burst in,
              the time will slide into the future. It is not problematic since
              the timestamp notion is opaque, and behaves like a revision number.
          """
          # XXX factorize code from memory and redis backends.
          is_specified = (record is not None and
                          modified_field in record or
                          last_modified is not None)
          if is_specified:
              # If there is a timestamp in the new record, try to use it.
              if last_modified is not None:
                  current = last_modified
              else:
                  current = record[modified_field]
          else:
              # Otherwise, use a new one.
              current = utils.msec_time()
  
          # Bump the timestamp only if it's more than the previous one.
          previous = self._timestamps[collection_id].get(parent_id)
          if previous and previous >= current:
              collection_timestamp = previous + 1
          else:
              collection_timestamp = current
  
          # In case the timestamp was specified, the collection timestamp will
          # be different from the updated timestamp. As such, we want to return
          # the one of the record, and not the collection one.
          if not is_specified:
              current = collection_timestamp
  
          self._timestamps[collection_id][parent_id] = collection_timestamp
          return current
  
      def create(self, collection_id, parent_id, record, id_generator=None,
                 unique_fields=None, id_field=DEFAULT_ID_FIELD,
                 modified_field=DEFAULT_MODIFIED_FIELD, auth=None):
          self.check_unicity(collection_id, parent_id, record,
                             unique_fields=unique_fields,
                             id_field=id_field,
                             for_creation=True)
  
          id_generator = id_generator or self.id_generator
          record = record.copy()
          _id = record.setdefault(id_field, id_generator())
          self.set_record_timestamp(collection_id, parent_id, record,
                                    modified_field=modified_field)
          self._store[collection_id][parent_id][_id] = record
          self._cemetery[collection_id][parent_id].pop(_id, None)
          return record
  
      def get(self, collection_id, parent_id, object_id,
              id_field=DEFAULT_ID_FIELD,
              modified_field=DEFAULT_MODIFIED_FIELD,
              auth=None):
          collection = self._store[collection_id][parent_id]
          if object_id not in collection:
              raise exceptions.RecordNotFoundError(object_id)
          return collection[object_id]
  
      def update(self, collection_id, parent_id, object_id, record,
                 unique_fields=None, id_field=DEFAULT_ID_FIELD,
                 modified_field=DEFAULT_MODIFIED_FIELD,
                 auth=None):
          record = record.copy()
          record[id_field] = object_id
  
          self.check_unicity(collection_id, parent_id, record,
                             unique_fields=unique_fields,
                             id_field=id_field)
  
          self.set_record_timestamp(collection_id, parent_id, record,
                                    modified_field=modified_field)
          self._store[collection_id][parent_id][object_id] = record
          return record
  
      def delete(self, collection_id, parent_id, object_id,
                 id_field=DEFAULT_ID_FIELD, with_deleted=True,
                 modified_field=DEFAULT_MODIFIED_FIELD,
                 deleted_field=DEFAULT_DELETED_FIELD,
                 auth=None, last_modified=None):
          existing = self.get(collection_id, parent_id, object_id)
          # Need to delete the last_modified field of the record.
          del existing[modified_field]
  
          self.set_record_timestamp(collection_id, parent_id, existing,
                                    modified_field=modified_field,
                                    last_modified=last_modified)
          existing = self.strip_deleted_record(collection_id,
                                               parent_id,
                                               existing)
  
          # Add to deleted items, remove from store.
          if with_deleted:
              deleted = existing.copy()
              self._cemetery[collection_id][parent_id][object_id] = deleted
          self._store[collection_id][parent_id].pop(object_id)
  
          return existing
  
      def purge_deleted(self, collection_id, parent_id, before=None,
                        id_field=DEFAULT_ID_FIELD,
                        modified_field=DEFAULT_MODIFIED_FIELD,
                        auth=None):
          num_deleted = len(self._cemetery[collection_id][parent_id].keys())
          if before is not None:
              kept = {key: value for key, value in
                      self._cemetery[collection_id][parent_id].items()
                      if value[modified_field] >= before}
          else:
              kept = {}
          self._cemetery[collection_id][parent_id] = kept
          return num_deleted - len(kept.keys())
  
      def get_all(self, collection_id, parent_id, filters=None, sorting=None,
                  pagination_rules=None, limit=None, include_deleted=False,
                  id_field=DEFAULT_ID_FIELD,
                  modified_field=DEFAULT_MODIFIED_FIELD,
                  deleted_field=DEFAULT_DELETED_FIELD,
                  auth=None):
          records = list(self._store[collection_id][parent_id].values())
  
          deleted = []
          if include_deleted:
              deleted = list(self._cemetery[collection_id][parent_id].values())
  
          records, count = self.extract_record_set(collection_id,
                                                   records + deleted,
                                                   filters, sorting,
                                                   id_field, deleted_field,
                                                   pagination_rules, limit)
  
          return records, count
  
  
  def get_unicity_rules(collection_id, parent_id, record, unique_fields,
                        id_field, for_creation):
      """Build filter to target existing records that violate the resource
      unicity rules on fields.
  
      :returns: a list of list of filters
      """
      rules = []
      for field in set(unique_fields):
          value = record.get(field)
  
          # None values cannot be considered unique.
          if value is None:
              continue
  
          filters = [Filter(field, value, COMPARISON.EQ)]
  
          if not for_creation:
              object_id = record[id_field]
              exclude = Filter(id_field, object_id, COMPARISON.NOT)
              filters.append(exclude)
  
          rules.append(filters)
  
      return rules
  
  
  def apply_sorting(records, sorting):
      """Sort the specified records, using cumulative python sorting.
      """
      result = list(records)
  
      if not result:
          return result
  
      first_record = result[0]
  
      def column(first, record, name):
          empty = first.get(name, float('inf'))
          return record.get(name, empty)
  
      for sort in reversed(sorting):
          result = sorted(result,
                          key=lambda r: column(first_record, r, sort.field),
                          reverse=(sort.direction < 0))
  
      return result
  
  
  def load_from_config(config):
      return Storage()
  