docstr: Emitter constructed with source argument
docstr_tok: ["Emitter","constructed","with","source","argument"]
code: def test_emitter_source(self):
      """Emitter constructed with source argument"""
      em = EventEmitter(source=self)
      ev = self.try_emitter(em, type="test_event")
      self.assert_result(
          event=ev,
          event_class=Event,
          source=self,
          type="test_event",
          sources=[self],
      )
  
      # overriding source should fail:
      try:
          ev = em(type="test_event", source=None)
          assert (
              False
          ), "Should not be able to specify source when emitting"
      except AttributeError:
          pass
  
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 2 for calling the TestEmitters obj's underlying member method code with example usage: obj.test_emitter_source() without return types