Checking your code is part of the development process. Developers rarely write code without verifying it. You are used to debugging programs as you write them. In fact, we devoted an entire chapter to debugging early in the course.
Your development process probably looks something like this:
But there's a better way to test your code, using automated tests. Automated tests actively test your code and help to remove the burden of manual testing. There are many types of automated tests. This chapter focuses on unit testing, which tests the smallest components (or units) of code. These are typically individual functions.
Before we dive into the how of unit testing, let's discuss the why.
Manual testing can eventually lead you to a complete, error-free program. Unit testing provides a better alternative.
This might sound familiar:
You write a program and manually test it. Thinking it is complete, you turn it in only to find that it has a bug or use case that you didn't consider.
The unit testing process helps avoid this by starting with a list of specific, clearly stated behaviors that the program should satisfy. The behaviors are then converted into automated tests that demonstrate program behavior and provide a framework for writing code that really works.
What about this situation?
You write feature #1 for a program. You then move on to feature #2. After finishing feature #2, you realize that your changes broke feature #1.
This is frustrating, right? Especially with larger programs, adding new features often causes unexpected problems in other parts of the code, potentially breaking the entire program. The introduction of such a bug is known as a regression.
If you have a collection of tests that can run quickly and consistently, you will know right away when a regression appears in your program. This allows you to identify and fix it more quickly.
One of the most powerful aspects of unit testing is that it allows us to clearly define program expectations. A good collection of unit tests can function as a set of statements about how the program should behave. You and others can read the tests and quickly get an idea of the specifics of program behavior.
Your coworker gives you a function that validates phone numbers, but doesn't provide much detail. Does it handle country codes? Does it require an area code? Does it allow parentheses around area codes? These details would be easily understood if the function had a collection of unit tests that described its behavior.
Code with a good, descriptive set of unit tests is sometimes called self-documenting code.
Remembering what your code does and why you structured it a certain way is easy for small programs. However, as the number of your projects increase and their size grows, the need for documentation becomes critical.
Documentation can be in the form of code comments or external text documents. These can be helpful, but have one major drawback which is that they can get out of date very quickly. Out dated, incorrect documentation is very frustrating for a user.
Properly designed unit tests are runnable documentation for your project. Because unit tests are runnable code that declares and verifies features, they can NEVER get out of sync with the updated code. If a feature is added or removed, the tests must be updated in order to make them pass.
Let's go ahead and write our first unit test!