class_docstr: A field requiring a callable has been set with a value that is not
  callable.
  
  .. versionadded:: 19.2.0
code: def __str__(self):
      return str(self.msg)
  
imports: from __future__ import annotations
  from typing import ClassVar
label_desc: Use image node_id 2 for calling the NotCallableError obj's underlying member method code with example usage: obj.__str__() and returns: str