docstr: The EventEmitter subclassing
docstr_tok: ["The","EventEmitter","subclassing"]
code: def test_emitter_subclass(self):
      """The EventEmitter subclassing"""
  
      class MyEmitter(EventEmitter):
          def _prepare_event(self, *args, **kwargs):
              ev = super(MyEmitter, self)._prepare_event(
                  *args, **kwargs
              )
              ev.test_tag = 1
              return ev
  
      em = MyEmitter(type="test_event")
      em.connect(self.record_event)
      self.result = None
      em()
      self.assert_result(test_tag=1)
  
imports: import unittest
  import copy
  import functools
  from vispy.util.event import Event, EventEmitter
  from vispy.testing import run_tests_if_main, assert_raises, assert_equal
label_desc: Use image node_id 7 for calling the TestEmitters obj's underlying member method code with example usage: obj.test_emitter_subclass() and returns: ev