code: def _from_deprecated_string(cls, serialized):
          
          if serialized.count('/') != 2:
              raise InvalidKeyError(cls, serialized)
  
          return cls(*serialized.split('/'), deprecated=True)
docs: Return an instance of `cls` parsed from its deprecated `serialized` form.
  
          This will be called only if :meth:`OpaqueKey.from_string` is unable to
          parse a key out of `serialized`, and only if `set_deprecated_fallback` has
          been called to register a fallback class.
  
          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`.