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



15 Answers

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?

mock and stub example

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.




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.




Stubs don't fail your tests, mock can.




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




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.



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!');
   }
}



A fake is a generic term that can be used to describe either a stub or a mock object (handwritten or otherwise), because they both look like the real object.

Whether a fake is a stub or a mock depends on how it’s used in the current test. If it’s used to check an interaction (asserted against), it’s a mock object. Otherwise, it’s a stub.

Fakes makes sure test runs smoothly. It means that reader of your future test will understand what will be the behavior of the fake object, without needing to read its source code (without needing to depend on external resource).

What does test run smoothly mean?
Forexample in below code:

 public void Analyze(string filename)
        {
            if(filename.Length<8)
            {
                try
                {
                    errorService.LogError("long file entered named:" + filename);
                }
                catch (Exception e)
                {
                    mailService.SendEMail("admin@hotmail.com", "ErrorOnWebService", "someerror");
                }
            }
        }

You want to test mailService.SendEMail() method, to do that you need to simulate an Exception in you test method, so you just need to create a Fake Stub errorService class to simulate that result, then your test code will be able to test mailService.SendEMail() method. As you see you need to simulate a result which is from an another External Dependency ErrorService class.




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.




See below example of mocks vs stubs using C# and Moq framework. Moq doesn't have a special keyword for Stub but you can use Mock object to create stubs too.

namespace UnitTestProject2
{
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using Moq;
    [TestClass]
    public class UnitTest1
    {
        /// <summary>
        /// Test using Mock to Verify that GetNameWithPrefix method calls Repository GetName method "once" when Id is greater than Zero
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsTwelve_GetNameCalledOnce()
        {
            // Arrange 
            var mockEntityRepository = new Mock<IEntityRepository>();
            mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));

            var entity = new EntityClass(mockEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(12);
            // Assert
            mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Once);
        }
        /// <summary>
        /// Test using Mock to Verify that GetNameWithPrefix method doesn't call Repository GetName method when Id is Zero
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsZero_GetNameNeverCalled()
        {
            // Arrange 
            var mockEntityRepository = new Mock<IEntityRepository>();
            mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));
            var entity = new EntityClass(mockEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(0);
            // Assert
            mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Never);
        }
        /// <summary>
        /// Test using Stub to Verify that GetNameWithPrefix method returns Name with a Prefix
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsTwelve_ReturnsNameWithPrefix()
        {
            // Arrange 
            var stubEntityRepository = new Mock<IEntityRepository>();
            stubEntityRepository.Setup(m => m.GetName(It.IsAny<int>()))
                .Returns("Stub");
            const string EXPECTED_NAME_WITH_PREFIX = "Mr. Stub";
            var entity = new EntityClass(stubEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(12);
            // Assert
            Assert.AreEqual(EXPECTED_NAME_WITH_PREFIX, name);
        }
    }
    public class EntityClass
    {
        private IEntityRepository _entityRepository;
        public EntityClass(IEntityRepository entityRepository)
        {
            this._entityRepository = entityRepository;
        }
        public string Name { get; set; }
        public string GetNameWithPrefix(int id)
        {
            string name = string.Empty;
            if (id > 0)
            {
                name = this._entityRepository.GetName(id);
            }
            return "Mr. " + name;
        }
    }
    public interface IEntityRepository
    {
        string GetName(int id);
    }
    public class EntityRepository:IEntityRepository
    {
        public string GetName(int id)
        {
            // Code to connect to DB and get name based on Id
            return "NameFromDb";
        }
    }
}



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.




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 :)




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




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.




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...




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.




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