Before we concentrate on unit tests, let's see what a test is, and what its purpose is.

A test is a piece of code whose purpose is to verify something in our system. It may be that we're calling a function passing two integers, that an object has a property called donald_duck, or that when you place an order on some API, after a minute you can see it dissected into its basic elements, in the database.

A test is typically composed of three sections:

• Preparation: This is where you set up the scene. You prepare all the data, the objects, and the services you need in the places you need them so that they are ready to be used.
• Execution: This is where you execute the bit of logic that you're checking against. You perform an action using the data and the interfaces you have set up in the preparation phase.
• Verification: This is where you verify the results and make sure they are according to your expectations. You check the returned value of a function, or that some data is in the database, some is not, some has changed, a request has been made, something has happened, a method has been called, and so on.

While tests usually follow this structure, in a test suite, you will typically find some other constructs that take part in the testing game:

• Setup: This is something quite commonly found in several different tests. It's logic that can be customized to run for every test, class, module, or even for a whole session. In this phase usually developers set up connections to databases, maybe populate them with data that will be needed there for the test to make sense, and so on.
• Teardown: This is the opposite of the setup; the teardown phase takes place when the tests have been run. Like the setup, it can be customized to run for every test, class or module, or session. Typically in this phase, we destroy any artefacts that were created for the test suite, and clean up after ourselves.
• Fixtures: They are pieces of data used in the tests. By using a specific set of fixture, outcomes are predictable and therefore tests can perform verifications against them.

In this chapter, we will use the pytest Python library. It is an incredibly powerful tool that makes testing much easier and provides plenty of helpers so that the test logic can focus more on the actual testing than the wiring around it. You will see, when we get to the code, that one of the characteristics of pytest is that fixtures, setup, and teardown often blend into one.