code: def _from_string(cls, serialized):
          
          try:
              def_key, aside_type = _split_keys_v2(serialized)
              return cls(DefinitionKey.from_string(def_key), aside_type)
          except ValueError as exc:
              raise InvalidKeyError(cls, exc.args)
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`.