Python unittest testing MongoDB randomly fails

Question

Having a weird problem with Python's unittest and PyMongo. The test randomly succeeds or fails:

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')

Class being tested:

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

Random behavior:

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)

These two results randomly happen for the same test as I rerun the test without changing anything neither in the class nor in the test.

All of this runs on my machine and I know for sure that while running the test, nobody else tinkers neither with MongoDB nor with the code.

What gives?

Answer 1

I strongly suspect the problem here is that you are not using "safe" mode for your writes.

By default MongoDB uses "fire and forget" mode. This means that the insert command is sent to the server, but the driver doesn't check for any server responses.

When you switch to "safe" mode, the driver will send the insert command and it will then send a second command getLastError. This second command will return when the server has actually committed the write.

Again, by default you are running in "fire and forget" mode, so there is indeed a potential race condition here. For unit tests you will need to run with "safe" mode on.

The function signature for insert is defined here. However, you should also be able to make the change at the Connection level so that each connection to the DB uses "safe" mode by default.