TST: add test for searching for removal with bare functions

IPython/core/tests/test_events.py

@@ -1,6 +1,7 b''
 from backcall import callback_prototype
 import unittest
 from unittest.mock import Mock
+import nose.tools as nt
 
 from IPython.core import events
 import IPython.testing.tools as tt
@@ -30,7 +31,18 b' class CallbackTests(unittest.TestCase):'
         self.em.unregister('ping_received', cb)
         self.em.trigger('ping_received')
         self.assertEqual(cb.call_count, 1)
-    
+
+    def test_bare_function_missed_unregister(self):
+        def cb1():
+            ...
+
+        def cb2():
+            ...
+
+        self.em.register('ping_received', cb1)
+        nt.assert_raises(ValueError, self.em.unregister, 'ping_received', cb2)
+        self.em.unregister('ping_received', cb1)
+
     def test_cb_error(self):
         cb = Mock(side_effect=ValueError)
         self.em.register('ping_received', cb)