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Spend any time learning about programming, and

you're gonna come across the acronym TDD, which stands for Test Driven Development.

In TDD, once you know what you're going to build, you write the test for

it before you write the code.

For some people, this works.

For others, it doesn't.

Don't feel bad if TDD doesn't make sense to you right now.

What's important is that you write tests, not whether you write the test before you

write the code, or whether you write tests for code that you've already written.

So what is a test?

In this course, when I talk about tests, I'm really talking about unit tests.

Well that brings up another question, huh?

What the heck is a unit?

A unit is a single feature or aspect of your code.

Say you have a class that represents a car.

It has a make, a model and a year.

It also has attributes like top speed and fuel capacity and maybe it has methods

that let you start and stop the car's engine or drive the car.

Each of those things, each attribute and each method, would be a unit and

each would need at least one test for it.

Since these tests correspond to a unit of code, we call them unit tests.

For example, our car might have a top speed attribute which

should always be set to a number.

We can write a test that sets it to a string like really fast, and

make sure our code raises an exception when that happens.

There are other types of tests too.

Regression tests make sure that previous mistakes don't happen again and

integration tests run your code through a battery of uses to make sure that

a certain process, one that uses multiple units, works from beginning to end.

For example, if you wanted to test the registration flow of a Flask app,

you'd have a test that loads the account creation page, fills out and

submits the form, follows the redirect, and

checks that the user model was created, and that you ended up on the sign in page.

We're gonna focus on unit tests in this course, but

you can use the tools we talk about to write all three kinds of tests.

Okay. Now that we know what a test is,

let's go look at how to write the simplest type of test, a dock test.