code: def _string_find(self, substr, start=None, end=None):
      
      if end is not None:
          raise NotImplementedError
      return ops.StringFind(self, substr, start, end).to_expr()
docs: Returns position (0 indexed) of first occurence of substring,
      optionally after a particular position (0 indexed)
  
      Parameters
      ----------
      substr : string
      start : int, default None
      end : int, default None
          Not currently implemented
  
      Returns
      -------
      position : int, 0 indexed