Python mock testing with mocker module

First of all install mocker:

easy_install mocker

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())

A Mocker instance is used for expectations record/replay. Here is our test (file mockerexample.py):

import unittest
from datetime import datetime

from mocker import Mocker, ANY, expect
from atm import Atm


class TestAtm(unittest.TestCase):

    def setUp(self):
        self._mocker = Mocker()
        self._mock_account = self._mocker.mock()
        self._atm = Atm()
        self._atm.signin(self._mock_account)

    def tearDown(self):
        self._atm.signout()
        self._mocker.restore()
        self._mocker.verify()

    def test_withdraw(self):
        # Arrange
        self._mock_account.deposit(150)
        self._mock_account.withdraw(100)
        self._mock_account.comission(0.5)
        expect(self._mock_account.balance(ANY))\
                .result(49.5)\
                .call(lambda d: self.assertTrue(d <= datetime.now()))
        self._mocker.replay()

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

        # Assert
        assert remaining_balance == 49.5

    def test_withdraw_insufficient_funds(self):
        # Arrange
        self._mock_account.deposit(50)
        expect(self._mock_account.withdraw(100))\
                .throw(ValueError('Insufficient funds'))
        self._mock_account.comission(0.1)
        expect(self._mock_account.balance(ANY))\
                .result(49.9)
        self._mocker.replay()

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

        # Assert
        assert remaining_balance == 49.9

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

Run tests:

python mockerexample.py

Read more about mocker here.