Mock and monkey patching example

README

Folder structure should be like

test_my_package.py
my_package/
          __init__.py
          my_module.py
          
Requieres mock to be available

Usage: python test_my_package

my_module

import os
import random

class MyClass(object):
    def __init__(self):
        self.cwd = None
        self.random_generator = random.random

    def get_current_work_dir(self):
        cwd = os.getcwd()
        self.cwd = cwd

    def get_random(self, n):
        return n*self.random_generator()

test_my_package

import unittest

import mock
import my_package

class MethodTests(unittest.TestCase):
    
        
    def test_get_current_work_dir(self):
        # Mock to be used with a large set of code, as is the os package
        self.mock_os = mock.patch('my_package.my_module.os').start()
        test_work_dir = 'foo'
        self.mock_os.getcwd.return_value = test_work_dir

        my_instance = my_package.my_module.MyClass()
        my_instance.get_current_work_dir()
        expected_work_dir = my_instance.cwd

        self.assertEquals(expected_work_dir, test_work_dir)       
    
    def test_get_random(self):
        # Monkey patching to replace the external call to random.random in random_generator method at MyClass
        my_instance = my_package.my_module.MyClass() 
        def return_random_number(): # This can be replaced with a lambda like function
            return 1
        my_instance.random_generator = return_random_number
        
        self.assertEquals(2, my_instance.get_random(2))

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