How to stub Python methods without Mock

13
votes

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 you can instantiate objects in your code and have them run the way you want, however, you can point methods on those objects to my own stubs defined in my tests - supposedly without mocks.

Is this true? I've tried to do it and can't quite get it working. How is this actually done? How do I stub a method in Python without a mocking library?

2
pythonunit-testingmockingstub

2 Answers

34
votes

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()
2
votes

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.