Mocking datetime.now() midway through a unittest.TestCase in Python 2.7 - python-unittest

Question

I'm trying to test a bit of debouncing logic - these are local unittests I run for a Google App Engine webapp, using the 2.7 runtime environment. All my other tests are going well but this one has me stumped!
def testThat_emailDebouncingWorks(self):
# Do something, it triggers an email.
doSomething()
self.assertEqual(emails_sent, 1)
# Do something again, the new email is debounced.
doSomething()
self.assertEqual(emails_sent, 1)
# After an hour, the emails should start working again...
mockWaitingAnHour()
doSomething()
self.assertEqual(emails_sent, 2)
# ... and so should the debouncing.
doSomething()
self.assertEqual(emails_sent, 2)
The file under test logs the time an email was sent using datetime.now(), then reruns datetime.now() on all future attempts and returns early if under an hour has elapsed.
There are two things going wrong:
I think the unittest library only added mock support in 3.X, and I'm not keen on updating my whole app.
Even if I was using 3.X, all the examples I see are about faking a datetime response for your entire test case (using a mock decorator above the test def). Whereas I want to change that behaviour midway through my test, not for the entire case.
Any tips? Thanks in advance!

Answer 1

Okay, I got to the bottom of it and wanted to document the answers for anyone who finds this on Google ;)
1. Enable mocking on AppEngine for Python 2.7
You need to follow the instructions for copying a third party library (in our case, "mock") from the official docs. It's worth noting that on Ubuntu, the suggested command:
pip install -t lib/ mock
Will fail. You'll get an error like this:
DistutilsOptionError: can't combine user with prefix, exec_prefix/home, or install_(plat)base
This is to do with a weird conflict with Ubuntu which seems to have gone unfixed for years, and you'll see a lot of people suggesting a virtualenv workaround. I added the --system flag instead:
pip install --system -t lib/ mock
and it worked fine. Remember to follow the rest of the instructions with appengine_config, and you should be set. "import mock" is a good way to check.
2. Mocking the datetime.now() call
My module under test uses:
from datetime import datetime
In my test module, import some stuff:
from mock import patch, Mock
import my_module #Also known as my_module.py
import datetime
Then the actual test case:
#patch.object(my_module, 'datetime', Mock(wraps=datetime.datetime))
def testThat_myModule_debouncesEmails(self):
fake_time = datetime.datetime.now()
# This is the first time the thing happened. It should send an email.
doSomething()
self.assertEqual(1, emails_sent)
# Five minutes later, the thing happens again. Should be debounced.
fake_time += datetime.timedelta(minutes=5)
my_module.datetime.now.return_value = fake_time
doSomething()
self.assertEqual(1, emails_sent)
# Another 56 minutes pass, the thing happens again. An hour has elapsed, so don't debounce.
fake_time += datetime.timedelta(minutes=56)
my_module.datetime.now.return_value = fake_time
doSomething()
self.assertEqual(2, emails_sent)
# Give it another 15 minutes to check the debouncing kicks back in.
fake_time += datetime.timedelta(minutes=15)
my_module.datetime.now.return_value = fake_time
doSomething()
self.assertEqual(2, emails_sent)
Hope this helps someone!