How to stub Python methods without Mock

Question

I\'m a C# dev moving into some Python stuff, so I don\'t know what I\'m doing just yet. I\'ve read that you don\'t really need Dependency Injection with Python. I\'ve been told

Answer 1

Here's a basic example. Note that the production getData() method is never called. It has been mocked out with a stub.

import unittest
class ClassIWantToTest(object):

    def getData(self):
        print "PRODUCTION getData called"
        return "Production code that gets data from server or data file"

    def getDataLength(self):
        return len(self.getData())

class TestClassIWantToTest(unittest.TestCase):

    def testGetDataLength(self):
        def mockGetData(self):
            print "MOCK getData called"
            return "1234"

        origGetData = ClassIWantToTest.getData
        try:
            ClassIWantToTest.getData = mockGetData
            myObj = ClassIWantToTest()
            self.assertEqual(4, myObj.getDataLength())
        finally:
            ClassIWantToTest.getData = origGetData

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

Answer 2

Python functions are first-class objects, so you can assign any function to the identifier:

class Person:
  def greetings(self):
    return "Hi!"

def happy_greetings(self):
  return "Hi! You're awesome!"

mike = Person()
mike.greetings()  # "Hi!"

Person.greetings = happy_greetings
mike.greetings()  # "Hi! You're awesome!"

Think of method identifier as a reference to some function. By changing the reference, Python interpreter will find and execute whatever it refers to. However, instead of doing this by hand, I suggest you using some mature module, like unittest.mock, since there are plenty of pitfalls like managing stub scope, etc.