testing aren't - What's the difference between a mock & stub?





mocks microservice (25)


A stub is an empty function which is used to avoid unhandled exceptions during tests:

function foo(){}

A mock is an artificial function which is used to avoid OS, environment or hardware dependencies during tests:

function foo(bar){ window = this; return window.toString(bar); }

In terms of assertions and state:

  • Mocks are asserted before an event or state change
  • Stubs are not asserted, they provide state before an event to avoid executing code from unrelated units
  • Spies are setup like stubs, then asserted after an event or state change
  • Fakes are not asserted, they run after an event with hardcoded dependencies to avoid state

References

I've read various articles about mocking vs stubbing in testing, including Martin Fowler's Mocks Aren't Stubs, but still don't understand the difference.




Foreword

There are several definitions of objects, that are not real. The general term is test double. This term encompasses: dummy, fake, stub, mock.

Reference

According to Martin Fowler's article:

  • Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
  • Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
  • Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
  • Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.

Style

Mocks vs Stubs = Behavioral testing vs State testing

Principle

According to the principle of Test only one thing per test, there may be several stubs in one test, but generally there is only one mock.

Lifecycle

Test lifecycle with stubs:

  1. Setup - Prepare object that is being tested and its stubs collaborators.
  2. Exercise - Test the functionality.
  3. Verify state - Use asserts to check object's state.
  4. Teardown - Clean up resources.

Test lifecycle with mocks:

  1. Setup data - Prepare object that is being tested.
  2. Setup expectations - Prepare expectations in mock that is being used by primary object.
  3. Exercise - Test the functionality.
  4. Verify expectations - Verify that correct methods has been invoked in mock.
  5. Verify state - Use asserts to check object's state.
  6. Teardown - Clean up resources.

Summary

Both mocks and stubs testing give an answer for the question: What is the result?

Testing with mocks are also interested in: How the result has been achieved?




I came across this interesting article by UncleBob The Little Mocker. It explains all the terminology in a very easy to understand manner, so its useful for beginners. Martin Fowlers article is a hard read especially for beginners like me.




A stub is a fake object built for test purposes. A mock is a stub that records whether expected calls effectively occurred.




If you compare it to debugging:

Stub is like making sure a method returns the correct value

Mock is like actually stepping into the method and making sure everything inside is correct before returning the correct value.




Stub is simple fake object. It just makes sure test runs smoothly.
Mock is smarter stub. You verify Your test passes through it.




In the codeschool.com course, Rails Testing for Zombies, they give this definition of the terms:

Stub

For replacing a method with code that returns a specified result.

Mock

A stub with an assertion that the method gets called.

So as Sean Copenhaver described in his answer, the difference is that mocks set expectations (i.e. make assertions, about whether or how they get called).







I have used python examples in my answer to illustrate the differences.

Stub - Stubbing is a software development technique used to implement methods of classes early in the development life-cycle. They are used commonly as placeholders for implementation of a known interface, where the interface is finalized or known but the implementation is not yet known or finalized. You begin with stubs, which simply means that you only write the definition of a function down and leave the actual code for later. The advantage is that you won't forget methods and you can continue to think about your design while seeing it in code. You can also have your stub return a static response so that the response can be used by other parts of your code immediately. Stub objects provide a valid response, but it's static no matter what input you pass in, you'll always get the same response:

class Foo(object):
    def bar1(self):
        pass

    def bar2(self):
        #or ...
        raise NotImplementedError

    def bar3(self):
        #or return dummy data
        return "Dummy Data"

Mock objects are used in mock test cases they validate that certain methods are called on those objects. Mock objects are simulated objects that mimic the behaviour of real objects in controlled ways. You typically creates a mock object to test the behaviour of some other object. Mocks let us simulate resources that are either unavailable or too unwieldy for unit testing.

mymodule.py:

import os
import os.path

def rm(filename):
    if os.path.isfile(filename):
        os.remove(filename)

test.py:

from mymodule import rm
import mock
import unittest

class RmTestCase(unittest.TestCase):
    @mock.patch('mymodule.os')
    def test_rm(self, mock_os):
        rm("any path")
        # test that rm called os.remove with the right parameters
        mock_os.remove.assert_called_with("any path")

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

This is a very basic example that just runs rm and asserts the parameter it was called with. You can use mock with objects not just functions as shown here, and you can also return a value so a mock object can be used to replace a stub for testing.

More on unittest.mock, note in python 2.x mock is not included in unittest but is a downloadable module that can be downloaded via pip (pip install mock).

I have also read "The Art of Unit Testing" by Roy Osherove and I think it would be great if a similar book was written using Python and Python examples. If anyone knows of such a book please do share. Cheers :)




I think the simplest and clearer answer about this question is given from Roy Osherove in his book The art of Unit Testing (page 85)

The easiest way to tell we’re dealing with a stub is to notice that the stub can never fail the test. The asserts the test uses are always against the class under test.

On the other hand, the test will use a mock object to verify whether the test failed or not. [...]

Again, the mock object is the object we use to see if the test failed or not.

That means if you are making assertions against the fake it means you are using the fake as a mock, if you are using the fake only to run the test without assertion over it you are using the fake as a stub.




Mock - A mock intercepts a call to a method or function (or a group of methods and functions like in the case of a mocked class). It is not an alternative to that method or function. In that interception, the mock can do whatever it wants, such as record the input and output, decide to short circuit the call, change the returned value, etc.

Stub - A stub is a valid full working implementation of a method or function (or group of methods and functions like in the case of a stubbed class) that has an identical interface/signature to the method, function or group of methods and functions it is stubbing for. The stubbed implementation will generally only do things that are acceptable within the context of a unit test, that means it won't do IO for example, while mimicking the behavior of the thing it is stubbing.




A Stub is an object that implements an interface of a component, but instead of returning what the component would return when called, the stub can be configured to return a value that suits the test. Using stubs a unit test can test if a unit can handle various return values from its collaborator. Using a stub instead of a real collaborator in a unit test could be expressed like this:

unit test --> stub

unit test --> unit --> stub

unit test asserts on results and state of unit

First the unit test creates the stub and configures its return values. Then the unit test creates the unit and sets the stub on it. Now the unit test calls the unit which in turn calls the stub. Finally the unit test makes assertions about the results of the method calls on the unit.

A Mock is like a stub, only it also has methods that make it possible determine what methods where called on the Mock. Using a mock it is thus possible to both test if the unit can handle various return values correctly, and also if the unit uses the collaborator correctly. For instance, you cannot see by the value returned from a dao object whether the data was read from the database using a Statement or a PreparedStatement. Nor can you see if the connection.close() method was called before returning the value. This is possible with mocks. In other words, mocks makes it possible to test a units complete interaction with a collaborator. Not just the collaborator methods that return values used by the unit. Using a mock in a unit test could be expressed like this:

unit test --> mock

unit test --> unit --> mock

unit test asserts on result and state of unit

unit test asserts on the methods called on mock

More Detail >> Here




let see Test Doubles:

  • Fake: Fakes are objects that have working implementations, but not the same as production one. Such as: in-memory implementation of Data Access Object or Repository.
  • Stub: Stub is an object that holds predefined data and uses it to answer calls during tests. Such as: an object that needs to grab some data from the database to respond to a method call.

  • Mocks: Mocks are objects that register calls they receive. In test assertion, we can verify on Mocks that all expected actions were performed. Such as: a functionality that calls e-mail sending service. for more just check this.




  • Stubs vs. Mocks
    • Stubs
      1. provide specific answers to methods calls
        • ex: myStubbedService.getValues() just return a String needed by the code under test
      2. used by code under test to isolate it
      3. cannot fail test
        • ex: myStubbedService.getValues() just returns the stubbed value
      4. often implement abstract methods
    • Mocks
      1. "superset" of stubs; can assert that certain methods are called
        • ex: verify that myMockedService.getValues() is called only once
      2. used to test behaviour of code under test
      3. can fail test
        • ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code
      4. often mocks interfaces



Stubs are used on methods with an expected return value which you setup in your test. Mocks are used on void methods which are verified in the Assert that they are called.




A Mock is just testing behaviour, making sure certain methods are called. A Stub is a testable version (per se) of a particular object.

What do you mean an Apple way?




I like the explanantion put out by Roy Osherove [video link].

Every class or object created is a Fake. It is a Mock if you verify calls against it. Otherwise its a stub.




I think the most important difference between them is their intentions.

Let me try to explain it in WHY stub vs. WHY mock

Suppose I'm writing test code for my mac twitter client's public timeline controller

Here is test sample code

twitter_api.stub(:public_timeline).and_return(public_timeline_array)
client_ui.should_receive(:insert_timeline_above).with(public_timeline_array)
controller.refresh_public_timeline
  • STUB: The network connection to twitter API is very slow, which make my test slow. I know it will return timelines, so I made a stub simulating HTTP twitter API, so that my test will run it very fast, and I can running the test even I'm offline.
  • MOCK: I haven't written any of my UI methods yet, and I'm not sure what methods I need to write for my ui object. I hope to know how my controller will collaborate with my ui object by writing the test code.

By writing mock, you discover the objects collaboration relationship by verifying the expectation are met, while stub only simulate the object's behavior.

I suggest to read this article if you're trying to know more about mocks: http://jmock.org/oopsla2004.pdf




Stub and Mock testing point of view:

  • Stub is dummy implementation done by user in static way mean i.e in Stub writing the implementation code. So it can not handle service definition and dynamic condition, Normally this is done in JUnit framework without using mocking framework.

  • Mock is also dummy implementation but its implementation done dynamic way by using Mocking frameworks like Mockito. So we can handle condition and service definition as dynamic way i.e. mocks can be created dynamically from code at runtime. So using mock we can implement Stubs dynamically.




Right from the paper Mock Roles, not Objects, by the developers of jMock :

Stubs are dummy implementations of production code that return canned results. Mock Objects act as stubs, but also include assertions to instrument the interactions of the target object with its neighbours.

So, the main differences are:

  • expectations set on stubs are usually generic, while expectations set on mocks can be more "clever" (e.g. return this on the first call, this on the second etc.).
  • stubs are mainly used to setup indirect inputs of the SUT, while mocks can be used to test both indirect inputs and indirect outputs of the SUT.

To sum up, while also trying to disperse the confusion from Fowler's article title: mocks are stubs, but they are not only stubs.




Reading all the explanations above, let me try to condense:

  • Stub: a dummy piece of code that lets the test run, but you don't care what happens to it.
  • Mock: a dummy piece of code, that you VERIFY is called correctly as part of the test.
  • Spy: a dummy piece of code, that intercepts some calls to a real piece of code, allowing you to verify calls without replacing the entire original object.



Here's a description of each one followed by with real world sample.

  • Dummy - just bogus values to satisfy the API.

    Example: If you're testing a method of a class which requires many mandatory parameters in a constructor which have no effect on your test, then you may create dummy objects for the purpose of creating new instances of a class.

  • Fake - create a test implementation of a class which may have a dependency on some external infrastructure. (It's good practice that your unit test does NOT actually interact with external infrastructure.)

    Example: Create fake implementation for accessing a database, replace it with in-memory collection.

  • Stub - override methods to return hard-coded values, also referred to as state-based.

    Example: Your test class depends on a method Calculate() taking 5 minutes to complete. Rather than wait for 5 minutes you can replace its real implementation with stub that returns hard-coded values; taking only a small fraction of the time.

  • Mock - very similar to Stub but interaction-based rather than state-based. This means you don't expect from Mock to return some value, but to assume that specific order of method calls are made.

    Example: You're testing a user registration class. After calling Save, it should call SendConfirmationEmail.

Stubs and Mocks are actually sub types of Mock, both swap real implementation with test implementation, but for different, specific reasons.




following is my understanding...

  • if you create test objects locally and feed your local service with that, you are using mock object. this will give a test for the method you implemented in your local service. it is used to verify behaviors

  • when you get the test data from the real service provider, though from a test version of interface and get a test version of the object, you are working with stubs the stub can have logic to accept certain input and give corresponding output to help you perform state verification...




To be very clear and practical:

Stub: A class or object that implements the methods of the class/object to be faked and returns always what you want.

Example in JavaScript:

var Stub = {
   method_a: function(param_a, param_b){
      return 'This is an static result';
   }
}

Mock: The same of stub, but it adds some logic that "verifies" when a method is called so you can be sure some implementation is calling that method.

As @mLevan says imagine as an example that you're testing a user registration class. After calling Save, it should call SendConfirmationEmail.

A very stupid code Example:

var Mock = {
   calls: {
      method_a: 0
   }

   method_a: function(param_a, param_b){
     this.method_a++; 
     console.log('Mock.method_a its been called!');
   }
}



The important thing is that you know what those terms mean to your colleagues. Different groups will have slightly varying definitions of what they mean when they say "full end-to-end" tests, for instance.

I came across Google's naming system for their tests recently, and I rather like it - they bypass the arguments by just using Small, Medium, and Large. For deciding which category a test fits into, they look at a few factors - how long does it take to run, does it access the network, database, filesystem, external systems and so on.

http://googletesting.blogspot.com/2010/12/test-sizes.html

I'd imagine the difference between Small, Medium, and Large for your current workplace might vary from Google's.

However, it's not just about scope, but about purpose. Mark's point about differing perspectives for tests, e.g. programmer vs customer/end user, is really important.





testing mocking stub