code: def magic_postpone_execution(self, line):
          
          line = line.strip().lower()
          if not line or line == 'true':
              self.interpreter.postpone_execution = True
          elif line == 'false':
              self.interpreter.postpone_execution = False
          else:
              stream_content = {'name': 'stderr', 'text': "unknwon value '{}'".format(line)}
              self.send_response(self.iopub_socket, 'stream', stream_content)
              return {'status': 'error',
                      'execution_count': self.execution_count,
                      'ename': ValueError.__name__,
                      'evalue': "unknwon value '{}'".format(line),
                      'traceback': '???'}
docs: Postpone execution of instructions until explicitly run
  
          Usage:
          Call this magic with `true` or nothing to postpone execution,
          or call with `false` to execute each instruction when evaluated.
          This defaults to True.
  
          Note that each cell is executed only executed after all lines in
          the cell have been evaluated properly.
  
          `%postpone_execution`
          or
          `%postpone_execution true`
          or
          `%postpone_execution false`