Python mock testing with mock module

Install mock module:

easy_install mock

Suppose you need to test withdraw operation in ATM (file atm.py):

from datetime import datetime

class Atm:
    def signin(self, account):
        self._account = account

    def signout(self):
        self._account = None

    def withdraw(self, amount):
        try:
            self._account.withdraw(amount)
            self._account.comission(amount * 0.005)
        except ValueError:
            self._account.comission(amount * 0.001)
        return self._account.balance(datetime.now())

After performing an action on Mock instance, you can make assertions about which methods / attributes were used and arguments they were called with. Here is our test (file mockexample.py):

from datetime import datetime
from atm import Atm
import unittest
from mock import Mock

class TestAtm(unittest.TestCase):

    def setUp(self):
        self._mock_account = Mock()
        self._atm = Atm()
        self._atm.signin(self._mock_account)

    def tearDown(self):
        self._atm.signout()

    def test_withdraw(self):
        # Arrange
        def balance(d):
            self.assertTrue(d >= datetime.now()) 
            return 49.5
        self._mock_account.balance.side_effect = balance

        # Act
        self._mock_account.deposit(150)
        remaining_balance = self._atm.withdraw(100)

        # Assert
        assert remaining_balance == 49.5
        self._mock_account.deposit.assert_called_with(150)
        self._mock_account.withdraw.assert_called_with(100)
        self._mock_account.comission.assert_called_with(0.5)

    def test_withdraw_insufficient_funds(self):
        # Arrange
        self._mock_account.withdraw\
                .side_effect = ValueError('Insufficient funds')
        self._mock_account.balance.return_value = 49.9

        # Act
        self._mock_account.deposit(50)
        remaining_balance = self._atm.withdraw(100)

        # Assert
        assert remaining_balance == 49.9
        self._mock_account.deposit.assert_called_with(50)
        self._mock_account.withdraw.assert_called_with(100)
        self._mock_account.comission.assert_called_with(0.1)

if __name__ == '__main__':
    unittest.main()

Run tests:

python mockexample.py

Read more about mock here.