code: def _resolve_subkeys(key, separator='.'):
      
      subkey = None
      if separator in key:
          index = key.index(separator)
          subkey = key[index + 1:]
          key = key[:index]
      return key, subkey
docs: Given a key which may actually be a nested key, return the top level
      key and any nested subkeys as separate values.
  
      Args:
          key (str): A string that may or may not contain the separator.
          separator (str): The namespace separator. Defaults to `.`.
  
      Returns:
          Tuple[str, str]: The key and subkey(s).