Python单元测试MongoDB随机失败

Python的unittest和PyMongo有一个奇怪的问题.测试随机成功或失败：

import unittest
from pymongo import Connection

from tractor import Tractor




class TestTractor(unittest.TestCase):
    def setUp(self):
        self.tractor = Tractor(1)

        self.mongo = Connection()
        self.db = self.mongo.tractor

        self.db.classes.remove({'name': {'$regex':'^test_'}})

        self.action_class_id = self.db.classes.insert({'name': 'test_action','metaclass': 'action'})
        self.object_class_id = self.db.classes.insert({'name': 'test_object','metaclass': 'object'})


    def tearDown(self):
        self.tractor = None



    def test_create_class(self):
        cid1 = self.tractor.create_action_class('test_create_action_class')
        cid2 = self.tractor.create_object_class('test_create_object_class')

        self.assertNotEqual(cid1,None)
        self.assertNotEqual(cid2,None)

        action_obj = self.db.classes.find_one({'_id': cid1})
        object_obj = self.db.classes.find_one({'_id': cid2})

        self.assertNotEqual(cid1,cid2)
        self.assertEqual(action_obj['_id'],cid1)
        self.assertEqual(object_obj['_id'],cid2)

        self.assertEqual(action_obj['name'],'test_create_action_class')
        self.assertEqual(object_obj['name'],'test_create_object_class')

正在测试的类：

from pymongo import Connection
from pymongo.objectid import ObjectId



class Tractor(object):
    def __init__(self,uid):
        self.uid = uid
        self.mongo = Connection()
        self.db = self.mongo.tractor


    # Classes

    def create_action_class(self,name):
        return self.db.classes.insert({'name': name,'attributes': [],'metaclass': 'action'})

    def create_object_class(self,'metaclass': 'object'})

随机行为：

silver@aregh-6930-lnx ~/projects/traction/tractor $python -m unittest discover
......ssEssssssssss
======================================================================
ERROR: test_create_class (tests.test_tractor.TestTractor)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/silver/projects/traction/tractor/tests/test_tractor.py",line 64,in test_create_class
    self.assertEqual(action_obj['_id'],cid1)
TypeError: 'NoneType' object is not subscriptable

----------------------------------------------------------------------
Ran 19 tests in 0.023s

FAILED (errors=1,skipped=12)

…

silver@aregh-6930-lnx ~/projects/traction/tractor $python -m unittest discover
......ss.ssssssssss
----------------------------------------------------------------------
Ran 19 tests in 0.015s

OK (skipped=12)

这两个结果随机发生在同一测试中,因为我重新运行测试而不改变类中和测试中的任何内容.

所有这些都在我的机器上运行,我确信在运行测试时,没有其他任何东西都没有使用MongoDB和代码.

是什么赋予了？