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[MUSIC]

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Spend any time learning about programming,
and

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

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

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it before you write the code.

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For some people, this works.

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For others, it doesn't.

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

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

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

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So what is a test?

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

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Well that brings up another question, huh?

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What the heck is a unit?

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

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Say you have a class that represents a
car.

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It has a make, a model and a year.

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

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

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

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each would need at least one test for it.

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

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

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should always be set to a number.

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

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make sure our code raises an exception
when that happens.

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There are other types of tests too.

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

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

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

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

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

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submits the form, follows the redirect,
and

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

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

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

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Okay.
Now that we know what a test is,

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