code: def _string_substr(self, start, length=None):
      
      op = ops.Substring(self, start, length)
      return op.to_expr()
docs: Pull substrings out of each string value by position and maximum
      length.
  
      Parameters
      ----------
      start : int
        First character to start splitting, indices starting at 0 (like
        Python)
      length : int, optional
        Maximum length of each substring. If not supplied, splits each string
        to the end
  
      Returns
      -------
      substrings : type of caller