old_contents: """Measurement entity model class."""
  
  from collections.abc import Iterable
  import re
  
  
  QUOTED_SLASH = re.compile("%2f", re.IGNORECASE)
  
  
  class Entity(dict):
      """Class to hold information about an individual measurement entity, e.g. one violation, or one security warning."""
  
      def __init__(self, key: str, **attributes) -> None:
          kwargs = dict(key=self.safe_entity_key(key))
          kwargs.update(**attributes)
          super().__init__(**kwargs)
  
      @staticmethod
      def safe_entity_key(key: str) -> str:
          """Return an escaped version of the key that is safe in URLs and as Mongo document key."""
          return re.sub(QUOTED_SLASH, "-", str(key).replace("/", "-").replace(".", "_"))
  
  
  class Entities(list[Entity]):
      """Class to hold a list of unique entities."""
  
      def __init__(self, entities: Iterable[Entity] = ()):
          """Extend to filter duplicate entities."""
          super().__init__()
          self.__keys: set[str] = set()  # Keep track of the keys in a set to make adding entities faster
          self.extend(entities)
  
      def __add__(self, other) -> "Entities":
          """Return the concatenation of the entities."""
          return self.__class__(super().__add__(other))
  
      def __getitem__(self, item):
          """Override to return an Entities slice or an Entity."""
          return self.__class__(super().__getitem__(item)) if isinstance(item, slice) else super().__getitem__(item)
  
      def append(self, entity: Entity) -> None:
          """Extend to only append the entity if it's not already in the list."""
          if (key := entity["key"]) not in self.__keys:
              self.__keys.add(key)
              super().append(entity)
  
      def extend(self, entities: Iterable[Entity]) -> None:
          """Extend to only add entities that aren't already in the list."""
          for entity in entities:
              self.append(entity)
  
new_contents: """Measurement entity model class."""
  
  from collections.abc import Iterable
  import re
  
  
  QUOTED_SLASH = re.compile("%2f", re.IGNORECASE)
  
  
  class Entity(dict):
      """Class to hold information about an individual measurement entity, e.g. one violation, or one security warning."""
  
      def __init__(self, key: str, **attributes) -> None:
          kwargs = dict(key=self.safe_entity_key(key))
          kwargs.update(**attributes)
          super().__init__(**kwargs)
  
      @staticmethod
      def safe_entity_key(key: str) -> str:
          """Return an escaped version of the key that is safe in URLs and as Mongo document key."""
          return re.sub(QUOTED_SLASH, "-", str(key).replace("/", "-").replace(".", "_"))
  
  
  class Entities(list[Entity]):
      """Class to hold a list of unique entities."""
  
      def __init__(self, entities: Iterable[Entity] = ()):
          """Extend to filter duplicate entities."""
          super().__init__()
          self.__keys: set[str] = set()  # Keep track of the keys in a set to make adding entities faster
          self.extend(entities)
  
      def __add__(self, other):
          """Return the concatenation of the entities."""
          return self.__class__(super().__add__(other))
  
      def __getitem__(self, item):
          """Override to return an Entities slice or an Entity."""
          return self.__class__(super().__getitem__(item)) if isinstance(item, slice) else super().__getitem__(item)
  
      def append(self, entity: Entity) -> None:
          """Extend to only append the entity if it's not already in the list."""
          if (key := entity["key"]) not in self.__keys:
              self.__keys.add(key)
              super().append(entity)
  
      def extend(self, entities: Iterable[Entity]) -> None:
          """Extend to only add entities that aren't already in the list."""
          for entity in entities:
              self.append(entity)
  
subject: Fix mypy error.