old_contents: _class_registry_cache = {}
  _field_list_cache = []
  
  
  def _import_class(cls_name):
      """Cache mechanism for imports.
  
      Due to complications of circular imports mongoengine needs to do lots of
      inline imports in functions.  This is inefficient as classes are
      imported repeated throughout the mongoengine code.  This is
      compounded by some recursive functions requiring inline imports.
  
      :mod:`mongoengine.common` provides a single point to import all these
      classes.  Circular imports aren't an issue as it dynamically imports the
      class when first needed.  Subsequent calls to the
      :func:`~mongoengine.common._import_class` can then directly retrieve the
      class from the :data:`mongoengine.common._class_registry_cache`.
      """
      if cls_name in _class_registry_cache:
          return _class_registry_cache.get(cls_name)
  
      doc_classes = ('Document', 'DynamicEmbeddedDocument', 'EmbeddedDocument',
                     'MapReduceDocument')
  
      # Field Classes
      if not _field_list_cache:
          from mongoengine.fields import __all__ as fields
          _field_list_cache.extend(fields)
          from mongoengine.base.fields import __all__ as fields
          _field_list_cache.extend(fields)
  
      field_classes = _field_list_cache
  
      queryset_classes = ('OperationError',)
      deref_classes = ('DeReference',)
  
      if cls_name == 'BaseDocument':
          from mongoengine.base import document as module
          import_classes = ['BaseDocument']
      elif cls_name in doc_classes:
          from mongoengine import document as module
          import_classes = doc_classes
      elif cls_name in field_classes:
          from mongoengine import fields as module
          import_classes = field_classes
      elif cls_name in queryset_classes:
          from mongoengine import queryset as module
          import_classes = queryset_classes
      elif cls_name in deref_classes:
          from mongoengine import dereference as module
          import_classes = deref_classes
      else:
          raise ValueError('No import set for: ' % cls_name)
  
      for cls in import_classes:
          _class_registry_cache[cls] = getattr(module, cls)
  
      return _class_registry_cache.get(cls_name)
  
  
new_contents: _class_registry_cache = {}
  _field_list_cache = []
  
  
  def _import_class(cls_name):
      """Cache mechanism for imports.
  
      Due to complications of circular imports mongoengine needs to do lots of
      inline imports in functions.  This is inefficient as classes are
      imported repeated throughout the mongoengine code.  This is
      compounded by some recursive functions requiring inline imports.
  
      :mod:`mongoengine.common` provides a single point to import all these
      classes.  Circular imports aren't an issue as it dynamically imports the
      class when first needed.  Subsequent calls to the
      :func:`~mongoengine.common._import_class` can then directly retrieve the
      class from the :data:`mongoengine.common._class_registry_cache`.
      """
      if cls_name in _class_registry_cache:
          return _class_registry_cache.get(cls_name)
  
      doc_classes = ('Document', 'DynamicEmbeddedDocument', 'EmbeddedDocument',
                     'MapReduceDocument')
  
      # Field Classes
      if not _field_list_cache:
          from mongoengine.fields import __all__ as fields
          _field_list_cache.extend(fields)
          from mongoengine.base.fields import __all__ as fields
          _field_list_cache.extend(fields)
  
      field_classes = _field_list_cache
  
      queryset_classes = ('OperationError',)
      deref_classes = ('DeReference',)
  
      if cls_name == 'BaseDocument':
          from mongoengine.base import document as module
          import_classes = ['BaseDocument']
      elif cls_name in doc_classes:
          from mongoengine import document as module
          import_classes = doc_classes
      elif cls_name in field_classes:
          from mongoengine import fields as module
          import_classes = field_classes
      elif cls_name in queryset_classes:
          from mongoengine import queryset as module
          import_classes = queryset_classes
      elif cls_name in deref_classes:
          from mongoengine import dereference as module
          import_classes = deref_classes
      else:
          raise ValueError('No import set for: ' % cls_name)
  
      for cls in import_classes:
          _class_registry_cache[cls] = getattr(module, cls)
  
      return _class_registry_cache.get(cls_name)
  
subject: fix styling flake8 error from CI