Posts tagged ‘London School TDD’

Last week I created a little quiz  and put a link to it on Twitter. I was interested to see whether the terminology around Test Doubles has standardized on Gerard Meszaro’s definitions, from his book “xUnit Test Patterns“, and I thought my twitter followers (I have over 1000 now!) might be able to tell me.

The quiz was taken by nearly 150 people, and overall, my conclusion is that Meszaro’s definitions are in general use today, (at least amongst people who follow me on Twitter). You can see a summary of the responses here. (I’ve closed the survey now, btw).

The first question was “Which of these is not a kind of Test Double”, and anyone who thought a “Suite” was a kind of test double clearly hasn’t got a clue, so I excluded their answers from my analysis. That was only a handful of responses though.

Looking at the remaining answers, between 70 – 85% share my understanding of what each kind of test double is. The scores are noticably a little lower for ‘Spy’, and actually I think my question on Spies was not very good. Jason Gorman kindly sent me a few tweets pointing out what he thought was unclear about it. The answer I was looking for to distinguish a Spy was “A Spy may raise an exception in the ‘Assert’ part of the test.”.  I was trying to articulate the difference between a “Spy” and a “Mock”, but I’ve personally only used Test Spy frameworks like Mockito and unittest.mock. Clearly I have more to learn.

I got the Mock question right, “A mock will fail the test if its methods are not called as expected”, but I thought a Spy was basically the same, except it fails the test in the “Assert” part instead of the “Act” part. Jason kindly pointed out that a Spy could be a Stub, a Fake, or even a decorated ‘real’ object. You don’t necessarily need to use a framework. The distinguishing feature of a Spy is that it records information about interactions, that your test can query later. So that was a good result from my quiz – I learnt something about Spies!

I also asked whether people had read Gerard Meszaro’s book, or my work-in-progress book, because I was interested to see if people would give better answers in this case. When I excluded all the people who said they hadn’t read either book, (about a third of responses), the scores improved significantly – over 80% agreed with me about the definitions of Mock, Stub and Fake. For Spies, on the other hand, the score was lower than for the group as a whole! That was clearly my fault…

Some people tried to argue that the distinction between the various kinds of test double is not interesting, that it doesn’t matter. I disagree. I think each kind has a different purpose and different situations when it is appropriate to use. If you can’t articulate what kind of test double you’re using, then there’s a very real danger you’re using it wrongly, or that better alternatives exist. Like – using several Mocks in the same test case, and finding it very fragile, when Stubs would be sufficient. Or – creating a new Stub for every test case, when it would be less work, and make for clearer tests, if you had a Fake that you could re-use in several.

So my conclusion from this quiz, and my other research on the subject, is that it’s worth using Meszaro’s definitions when discussing Test Doubles. A lot of programmers have a good working understanding of them, and do know the difference between a Stub and a Mock. So that’s very encouraging! One of the reasons Meszaros invented the “Test Double” terminology was to clear up the confusion that reigned at the time. It seems to me we’re in a much better position today because of his work. I do think there is more to do, though, I still see people using Test Doubles badly. Which is partly the motivation for my new book, which is about various techniques in Test Driven Development, not just Test Doubles actually. I’d better go and do some work on the section on Spies…

This is the third post in a series about London School TDD. The first one is here, introducing the topic. The second post discusses “Outside-In Development with Double-Loop TDD”. In this post I’d like to talk about the second difference I see between Classic and London School TDD, which is to do with your style of Object Oriented Design.

“Different design styles have different techniques that are most applicable for test-driving code written in those styles, and there are different tools that help you with those techniques…

That’s what we  … designed JMock to do …
“Tell, Don’t Ask” object-oriented design.”

— Nat Pryce, in an email to a discussion forum.

That quote explains the objective Nat et al had when designing JMock, and I think it shows  that London School TDD is actually a school of design as much as a testing technique. Let’s take a closer look at this way of designing objects.

Tell, Don’t Ask

“Tell, Don’t Ask” Object Oriented Design is about having Cohesive objects that hide their internal workings. If your objects obey the Law of Demeter, that’s a good start, it means they hide their inner workings and don’t talk to objects far away on the object graph. It reduces Coupling in your system, which should make for better maintainability.

In their book “Growing Object Oriented Software, Guided by Tests”, Freeman & Pryce actually define “Tell, Don’t Ask” as the same as following the Law of Demeter (p17). Then they go on with several chapters about their design style, expanding far beyond simply “following the Law of Demeter”. It’s well worth a read, here’s a sample:

“… we focus our design effort on how the objects collaborate … obviously, we want to achieve a well-designed class structure, but we think the communication patterns between objects are more important.”

— Freeman & Pryce, GOOS, p58

Message passing vs Types with Data

So it’s basically about how you view your objects. Do you see them primarily in terms of sending and receiving messages to other objects in order to get stuff done? Or are you more focussed on the data your objects look after and the class of objects they are part of?

In the diagram below you can see an object is defined in terms of which messages it sends and receives:      london_school_008

This diagram shows the same object, but with a focus on data rather than messages:


If you have a “message” focus, you’ll be concerned with defining protocols and interfaces. You’ll worry about which collaborators will be needed to process a particular message. If you have a “data” focus, you’ll be interested in checking your object goes through particular state transitions. You’ll check it makes correct calculations based on its data, and hides whether the result is cached or calculated.

In my first post I talked about the three ways to verify object behaviour. In Classic TDD, the most popular way to write your assert is to check the state of the object you’re testing, or a collaborator, using a public API. This naturally leads you to design objects that are more type-oriented, with the emphasis on class relationships.

In London School TDD, the favoured way to write your assert is to use a mock and check a particular interaction happened, or in other words, a particular message was passed. This is because you favour a design where objects don’t reveal much at all about their data – your system is all about the interactions.

Dependencies and Collaborators

In a message-oriented design, it’s natural to want to specify which collaborators a particular object needs in order to get something done, and what messages it will send them. It’s part of the public specification of an object, and natural to pin down in a test case using mocks. If you instead check your object via a method that lets you query its state, it could expose details that might stop you refactoring the internals later. This leads you to prefer to check your messages, rather than state and data.

If you have a more type-oriented design, you may want to hide the fact you’re storing data accross several objects, or delegating certain calculations to other objects. Those dependencies aren’t part of the public specification, what matters is the end result. If you start exposing these interactions in your test via mocks, you’ll end up with brittle tests that hinder a subsequent redistribution of responsibilities between an object and its dependents. This leads you to prefer to check state and data, rather than interactions.

Comparing the two styles

In these articles I’ve tried to draw each style of TDD to an extreme in order to emphasize the differences. Of course, in practice, a competent developer will use the style most appropriate to the situation she finds herself in. She may use both styles while developing different pieces of the same system. In my next post, I’d like to illustrate this with a small example.

In my last post, I started talking about London School TDD, and the two features of it that I think distinguish it from Classic TDD. The first was Outside-In development with Double Loop TDD, which I’d like to talk more about in this post. The second was “Tell, Don’t Ask” Object Oriented Design. I’ll take that topic up in my next post.

Double Loop TDD


When you’re doing double loop TDD, you go around the inner loop on the timescale of minutes, and the outer loop on the timescale of hours to days. The outer loop tests are written from the perspective of the user of the system. They generally cover thick slices of functionality, deployed in a realistic environment, or something close to it. In my book I’ve called this kind of test a “Guiding Test”, but Freeman & Pryce call them “Acceptance Tests”. These tests should fail if something the customer cares about stops working – in other words they provide good regression protection. They also help document what the system does. (See also my article “Principles for Agile Automated Test Design“).

I don’t think Double Loop TDD is unique to the London School of TDD, I think Classic TDDers do it too. The idea is right there in Kent Beck’s first book about eXtreme Programming. What I think is different in London School, is designing Outside-In, and the use of mocks to enable this.

Designing Outside-In

If you’re doing double loop TDD, you’ll begin with a Guiding Test that expresses something about how a user wants to interact with your system. That test helps you identify the top level function or class that is the entry point to the desired functionality, that will be called first. Often it’s a widget in a GUI, a link on a webpage, or a command line flag.

With London School TDD, you’ll often start your inner loop TDD by designing the class or method that gets called by that widget in the GUI, that link on the webpage, or that command line flag. You should quickly discover that this new piece of code can’t implement the whole function by itself, but will need collaborating classes to get stuff done.


The user looks at the system, and wants some functionality. This implies a new class is needed at the boundary of the system. This class in turn needs collaborating classes that don’t yet exist.

The collaborating classes don’t exist yet, or at least don’t provide all the functionality you need. Instead of going away and developing these collaborating classes straight away, you can just replace them with mocks in your test. It’s very cheap to change mocks and experiment until you get the the interface and the protocol just the way you want it. While you’re designing a test case, you’re also designing the production code.


You replace collaborating objects with mocks so you can design the interface and protocol between them.

When you’re happy with your design, and your test passes, you can move down the stack and start working on developing the implementation of one of the collaborating classes. Of course, if this class in turn has other collaborators, you can replace them with mocks and design these interactions too. This approach continues all the way through the system, moving through architectural layers and levels of abstraction.


You’ve designed the class at the boundary of the system, and now you design one of the collaborating classes, replacing its collaborators with mocks.

This way of working lets you break a problem down into manageable pieces, and get each part specified and tested before you move onto the next part. You start with a focus on what the user needs, and build the system from the “outside-in”, following the user interaction through all the parts of the system until the guiding test passes. The Guiding Test will not usually replace parts of the system with mocks, so when it passes you should be confident you’ve remembered to actually implement all the needed collaborating classes.

Outside-In with Classic TDD

A Classic TDD approach may work outside-in too, but using an approach largely without mocks. There are various strategies to cope with the fact that collaborators don’t exist yet. One is to start with the degenerate case, where nothing much actually happens from the user’s point of view. It’s some kind of edge case where the output is much simpler than in the normal or happy-path case. It lets you build up the structure of the classes and methods needed for a simple version of the functionality, but with basically empty implementations, or simple faked return values. Once the whole structure is there, you flesh it out, perhaps working inside-out.

Another way to do this in Classic TDD is to start writing the tests from the outside-in, but when you discover you need a collaborating class to be implemented before the test will pass, comment out that test and move down to work on the collaborator instead. Eventually you find something you can implement with collaborators that already exist, then work your way up again.

A Classic TDD approach will often just not work outside-in at all. You start with one of the classes nearer the heart of the system. You’ll pick something that can be fully implemented and tested using collaborating classes that already exist.  Often it’s a class in the central domain model of the application. When that is done, you continue to develop the system from the heart towards the outside, adding new classes that build on one another. Because you’re using classes that already exist, there is little need for using mocks. Eventually you find you’ve built all the functionality needed to get the Guiding Test to pass.

Pros and Cons

I think there’s a definite advantage to working outside-in, it keeps your focus on what the user really needs, and helps you to build something useful, without too much gold-plating. I think it takes skill and discipline to work this way with either Classic or London School. It’s not easy to see how to break down a piece of functionality into incremental pieces that you can develop and design step-by-step. If you work from the heart outwards, there is a danger you’ll build more than you need for what the user wants, or that you’ll get to the outside, discover it doesn’t “fit”, and have to refactor your work.

Assuming you are working outside-in, though, one difference seems to me to be in whether you write faked implementations in the actual production code, or in mocks. If you start with fakes in the production code, you’ll gradually replace them with real functionality. If you put all the faked functionality into mocks, they’ll live with the test code, and remain there when the real functionality is implemented. This could be useful for documentation, and will make your tests continue to execute really fast.

Having said that, there some debate about the maintainability of tests that use a lot of mocks. When the design changes, it can be prohibitive to update all the mocks as well as the production code. Once the real implementations are done, maybe the inner-loop tests should just be deleted? The Guiding Test could provide all the regression protection you need, and maybe the tests that helped you with your original design aren’t useful to keep? I don’t think it’s clear-cut actually. From talking to London School proponents, they don’t seem to delete all the tests that use mocks. They do delete some though.

I’m still trying to understand these issues and work out in what contexts London School TDD brings the most advantage. I hope I’ve outlined what I see as the differences in way of working with outside-in development. In my next post I look at how London School TDD promotes “Tell, Don’t Ask” Object Oriented Design.

Recently I’ve become quite interested in the London School of TDD. I blogged before about my experiences doing Luca Minudel’s exercises, in my post “SOLID Principles and TDD“. Since I wrote that, I’ve read Steve Freeman and Nat Pryce’s book “Growing Object Oriented Software, Guided by Tests” and practiced doing some code katas in this style. In my experience there is a lot of confusion around how to use Mocks, and I found it enlightening to see  how the people who invented the technique actually use them.

My current thinking is that there are at least these two areas where London School TDD differs from Classic TDD:

  • Outside-In development with Double-Loop TDD
  • “Tell, Don’t Ask” Object Oriented Design

London School practitioners use Mock Objects as a tool for achieving both. Let’s look a little more about what Mocks are for.

Verifying behaviour

A test case often has three parts: “Arrange – Act – Assert”. In the second edition of his book The Art of Unit Testing, Roy Osherove points out that in the “Assert” part of a test there are three ways to ensure the class you’re testing is behaving correctly. After “Arrange” and “Act”, you can:

  • Check the return value, or an exception.
  • Check the state of the object, or the state of a collaborator.
  • Check the object correctly interacts with a collaborator.

This last form of assertion is generally done using a Mock Object. With an ordinary Mock you set it up in advance to check for a particular interaction, with a Spy, you check after the fact. In either case, you’re asserting an interaction happens correctly. You’re checking a particular object received a particular method call, and you can be more or less strict about the precise details of arguments and numbers of invocations.

In Classic TDD, whenever possible you check a return value or exception. If you’re testing a void method, then you usually take the second option and check state. Only if the other options are really unattractive do you ever turn to using a mock. It’s the last choice.

With London School TDD, the option of using a mock is chosen much more often. You’ll still check return values or object states where that makes sense, but using a mock is often an attractive option. This is because using a mock helps you to both develop your system Outside-In, and to design your objects in an “Tell, Don’t Ask” manner.

Mocks used badly

I think one reason that using a mock object is often the last choice in classic TDD, is because it’s so easy to get into trouble when using them. You should be using mocks to help you improve your design, but all too often the design is bad, and the mocks are either hiding that, or getting in the way.  I sometimes see tests with an enormous “Arrange” part, specifying half a dozen different mocks before they’ve even started calling any functionality. Such a test is bound to be brittle, and could hinder your refactoring to a better design.

The article “Eliminate most Mocks from Unit Tests” by Arlo Belshee gives an example of using a mock to compensate for bad design, and he has several other articles in the series. I think Arlo is largely criticising poor use of mocks actually, rather than London School TDD itself.

It seems to me that you can abuse any technique, and Object Oriented Design is actually very difficult. Steve Freeman has said “No tool nor technique can survive inadequately trained developers“. London School TDD is a design technique that is not easy to master. I talk more about this in my next posts “Outside-In Development with Double Loop TDD“, and “Tell, Don’t Ask” Object Oriented Design.