code: def str_passthrough(obj):
      """
      Used to be cast_unicode, add this temporarily to make sure no further breakage.
      """
      if not isinstance(obj, str):
          raise AssertionError
      return obj
  
file_path: nbformat/nbformat/v3/nbbase.py
imports: from __future__ import annotations
  import warnings
  from nbformat._struct import Struct
label_desc: Use image node_id 2 for calling a global function with example usage: str_passthrough(obj) and returns: obj