code: def _resolve_subkeys(key, separator="."):
      
      parts = key.split(separator, 1)
  
      if len(parts) > 1:
          return parts
      else:
          return parts[0], None
docs: Resolve a potentially nested key.
  
      If the key contains the ``separator`` (e.g. ``.``) then the key will be
      split on the first instance of the subkey::
  
         >>> _resolve_subkeys('a.b.c')
         ('a', 'b.c')
         >>> _resolve_subkeys('d|e|f', separator='|')
         ('d', 'e|f')
  
      If not, the subkey will be :data:`None`::
  
          >>> _resolve_subkeys('foo')
          ('foo', None)
  
      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).