Implement remove() for _empty_collection

Fixed regression caused in 1.2.3 due to fix from :ticket:`4181` where
the changes to the event system involving :class:`.Engine` and
:class:`.OptionEngine` did not accommodate for event removals, which
would raise an ``AttributeError`` when invoked at the class
level.

Change-Id: I1c9083829d74dd710716d28b0eaca4fa15e86313
Fixes: #4190

1 files changed, 9 insertions, 0 deletions

diff --git a/test/engine/test_execute.py b/test/engine/test_execute.py
index e0727f770..2d602fa12 100644
--- a/test/engine/test_execute.py
+++ b/test/engine/test_execute.py
@@ -1399,6 +1399,15 @@ class EngineEventsTest(fixtures.TestBase):
 
         eq_(canary, ["l1", "l2", "l3", "l4", "l1", "l2", "l3"])
 
+        canary[:] = []
+
+        event.remove(Engine, "before_execute", l1)
+        event.remove(eng1, "before_execute", l4)
+        event.remove(eng, "before_execute", l3)
+
+        eng1.execute(select([1])).close()
+        eq_(canary, ["l2"])
+
     @testing.requires.ad_hoc_engines
     def test_cant_listen_to_option_engine(self):
         from sqlalchemy.engine import base