This post contains a step-by-step example of a refactoring session guided by tests. When dealing with untested or legacy code refactoring is dangerous and tests can help us do it the right way, minimizing the amount of bugs we introduce, and possibly completely avoiding them.
Refactoring is not easy. It requires a double effort to understand code that others wrote, or that we wrote in the past, and moving around parts of it, simplifying it, in one word improving it, is by no means something for the faint-hearted. Like programming, refactoring has its rules and best practices, but it can be described as a mixture of technique, intuition, experience, risk.
Programming, after all, is craftsmanship.
The starting point
The simple use case I will use for this post is that of a service API that we can access, and that produces data in JSON format, namely a list of elements like the one shown here
This is my solution of the challenge posted here. As I stressed in that post, this is just one possible solution, and not even necessarily the best one. I provide it to show how I managed to solve the tests and how I worked in a TDD way.
Speaking of TDD I realised that I hadn't followed it very strictly, as sometimes I wrote more code than needed, usually forecasting future changes. I do not believe in a inflexible and uncompromising application of rules, so I do not consider this a big issue, as long as the result is a working code that is not blatantly overengineered.
Level 1 - End of file
The base class to pass the test leverages the provided
text_buffer.TextBuffer class, that exposes a
load() method, directly composed here to
CalcLexer.load(). As the test is not providing a text the easiest solution is just to return the tested
Writing an interpreter or a compiler is usually considered one of the greatest goals that a programmer can achieve, and with good reason. I do not believe the importance of going through this experience is primarily due to its difficulty. After all, writing an efficient compiler is difficult, but the same is true for a good web framework, or a feature-rich editor.
Being able to write an interpreter is a significant skill mainly because of its recursive (or self-referring) nature. Think about it: you use a language to write a new language. And this new language, if it becomes sufficiently rich, can eventually be used to create its own compiler.
A language can be used to write the program that executes that same language.
Didn't this last sentence fire you with enthusiasm? It makes me eager to start!
Compilers have been the subject of academic research since the 50s, with the works of ... more
In the first post I introduced you to Python mocks, objects that can imitate other objects and work as placeholders, replacing external systems during unit testing. I described the basic behaviour of mock objects, the
side_effect attributes, and the
In this post I will briefly review the remaining
assert_* methods and some interesting attributes that allow to check the calls received by the mock object. Then I will introduce and exemplify patching, which is a very important topic in testing.
Other assertions and attributes
As already stressed in the two introductory posts on TDD (you can find them here) testing requires to write some code that uses the functions and objects you are going to develop. This means that you need to isolate a given (external) function that is part of your public API and demonstrate that it works with standard inputs and in edge cases.
For example, if you are going to develop an object that stores percentages (such as for example poll results), you should test the following conditions: the class can store a standard percentage such as 42%, the class shall give an error if you try to store a negative percentage, the class shall give an error if you store a percentage greater than 100%.
Tests shall be idempotent and isolated. Idempotent in mathematics and computer science identifies a process that can be run multiple times without changing the status of the system. Isolated means that a test shall not change its behaviour depending on ... more
In the first part of this small series I introduced you to TDD with Python by means of the powerful
py.test package. We developed together a simple library which provides a
Binary class that is a bit more useful than the default binary representation that Python provides with the
bin() builtin function.
In this part I'll go on with the development of the library, discussing the implementation of a binary number with a fixed size, which is a very interesting and useful matter, being the foundation of the computer we are working with. Fixed-size binaries may also represent negative numbers with the two's complement technique, and this will be an important point to test.
You may happen to dislike some decisions about the interface or the behaviour of the resulting class. Since this post is just a way to show you a concrete TDD session you are totally free to change the tests and to come up with a ... more
If you are eager to learn some Python and do not know how to start, this post may give you some hints. I will develop a very simple Python package from scratch, exemplifying some Object-oriented Programming (OOP) techniques and concepts, and using a Test-Driven Development (TDD) approach.
The package will provide some classes to deal with binary numbers (see the Rationale section), but remember that it is just a toy project. Nothing in this package has been designed with performance in mind: it wants to be as clear as possible.
Binary numbers are rather easy to understand, even if becoming familiar with them requires some time. I expect you to have knowledge of the binary numeral system. If you need to review them just take a look at the Wikipedia entry or one of the countless resources on Internet.
The package we are going to write will provide a class that represents binary numbers (