code: def _from_string(cls, serialized):
          
          if ':' not in serialized:
              raise InvalidKeyError(
                  "BlockTypeKeyV1 keys must contain ':' separating the block family from the block_type.", serialized)
          family, __, block_type = serialized.partition(':')
          return cls(family, block_type)
docs: Return an instance of `cls` parsed from its `serialized` form.
  
          Args:
              cls: The :class:`OpaqueKey` subclass.
              serialized (unicode): A serialized :class:`OpaqueKey`, with namespace already removed.
  
          Raises:
              InvalidKeyError: Should be raised if `serialized` is not a valid serialized key
                  understood by `cls`.