class_docstr: Base class to build BackgroundJob classes.
  
  The derived classes must implement:
  
  - Their own __init__, since the one here raises NotImplementedError.  The
    derived constructor must call self._init() at the end, to provide common
    initialization.
  
  - A strform attribute used in calls to __str__.
  
  - A call() method, which will make the actual execution call and must
    return a value to be held in the 'result' field of the job object.
code: def __repr__(self):
      return "<BackgroundJob #%d: %s>" % (self.num, self.strform)
  
imports: from __future__ import print_function
  import sys
  import threading
  from IPython import get_ipython
  from IPython.core.ultratb import AutoFormattedTB
  from logging import error
  from IPython.utils.py3compat import string_types
label_desc: Use image node_id 4 for calling the BackgroundJobBase obj's underlying member method code with example usage: obj.__repr__() and returns: unknown