RUNNING TESTS IN
ISOLATION USING MOCK
OBJECTS
Alberto Rodriguez Peon
IT-PES-PS
Motivating example: ai-bs-vm
Command-line tool to create Puppetized VMs
How to test this? Isolating it with mocks
What is a mock, then?
A mock is a “stunt double” used in test time to
eliminate a single dependency
Image from: http://zeroturnaround.com/rebellabs/how-to-mock-up-your-unit-test-environment-to-createalternate-realities/
How does it look like?
def test_play(self):
# game.play calls game._toss_coin which returns
# ‘heads’ or ‘tails’ randomly
assert self.game.play(guess=‘heads’) == ‘You win!’
assert self.game.play(guess=‘tails’) == ‘You lose!’
This test case is broken
How does it look like?
import mock
@patch.object(Game, ‘_toss_coin’, return_value=‘heads’)
def test_play(self):
# game.play calls game._toss_coin which returns
# ‘heads’ or ‘tails’ randomly
assert self.game.play(guess=‘heads’) == ‘You win!’
assert self.game.play(guess=‘tails’) == ‘You lose!’
What should I mock?
Every operation which:
 Is slow (A query to a database)
 Is non-deterministic (Obtaining the current date and time)
 Can trigger hard to reproduce results (A timeout)
Take-away message
Mocking makes possible to test untestable
code by isolating it from its dependencies