after_flush_postexec events skipped when removing handlers

gistfile1.txt

python -m unittest test_events
.F...F
======================================================================
FAIL: testDoubleRemove (sqlalchemy_mptt.tests.test_flush.FlushingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "sqlalchemy_mptt/tests/test_flush.py", line 60, in testDoubleRemove
    self.assertEqual(self.calls, 2)
AssertionError: 1 != 2

======================================================================
FAIL: testTripleRemove (sqlalchemy_mptt.tests.test_flush.FlushingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "sqlalchemy_mptt/tests/test_flush.py", line 67, in testTripleRemove
    self.assertEqual(self.calls, 3)
AssertionError: 2 != 3

----------------------------------------------------------------------
Ran 6 tests in 0.046s

FAILED (failures=2)

test_events.py

import unittest

from sqlalchemy import create_engine, Column, Integer, String, event
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class TestModel(Base):
    __tablename__ = 'test_model'

    id = Column(Integer, primary_key=True)
    name = Column(String, unique=True)


class Receiver(object):
    def __init__(self, instance, remove):
        self.instance = instance
        self.remove = remove

    def __call__(self, session, context):
        self.instance.calls += 1
        if self.remove:
            event.remove(self.instance.session, 'after_flush_postexec', self)

    @classmethod
    def register(cls, instance, remove):
        self = cls(instance, remove=remove)
        event.listen(instance.session, 'after_flush_postexec', self)


class FlushingTest(unittest.TestCase):
    base = Base
    model = TestModel

    def setUp(self):
        self.engine = create_engine('sqlite:///:memory:')
        Session = sessionmaker(bind=self.engine)
        self.session = Session()
        self.base.metadata.create_all(self.engine)
        self.calls = 0

    def tearDown(self):
        self.base.metadata.drop_all(self.engine)

    def flush(self):
        self.session.add(self.model(name='test'))
        self.session.flush()

    def testSingleRemove(self):
        Receiver.register(self, remove=True)
        self.flush()
        self.assertEqual(self.calls, 1)

    def testDoubleRemove(self):
        Receiver.register(self, remove=True)
        Receiver.register(self, remove=True)
        self.flush()
        self.assertEqual(self.calls, 2)

    def testTripleRemove(self):
        Receiver.register(self, remove=True)
        Receiver.register(self, remove=True)
        Receiver.register(self, remove=True)
        self.flush()
        self.assertEqual(self.calls, 3)

    def testSingleNoRemove(self):
        Receiver.register(self, remove=False)
        self.flush()
        self.assertEqual(self.calls, 1)

    def testDoubleNoRemove(self):
        Receiver.register(self, remove=False)
        Receiver.register(self, remove=False)
        self.flush()
        self.assertEqual(self.calls, 2)

    def testTripleNoRemove(self):
        Receiver.register(self, remove=False)
        Receiver.register(self, remove=False)
        Receiver.register(self, remove=False)
        self.flush()
        self.assertEqual(self.calls, 3)