docs: def _valid_key(self, key):
          """Return True if supplied key string serves as a valid attribute name: alphanumeric strings
          beginning with a letter.  Leading underscore not allowed.  Also test for conflict with protected
          attribute names (e.g. dict class instance methods).
          """
          if not isinstance(key, str):
              return False
          elif hasattr({}, key):
              return False
          elif key in self._special_names:
              return False
          else:
              return self._valid_key_pattern.match(key)