Your First Test

What a test is

You already know how to write Python functions, you did it all through Python for Beginners. A test is just more Python: a small piece of code that runs your function and checks the answer automatically.

The checking tool is assert, a built-in statement you may remember from Advanced Python. It works like this:

• assert condition does nothing if the condition is True
• if the condition is False, it raises an AssertionError and the program stops

That is the whole trick. Instead of running your code, staring at the output, and judging it by eye, you write the judgment down as code. The computer then re-checks it every time, in milliseconds, forever.

This habit is not optional in real jobs. Professional teams run thousands of these automated checks on every change, and at most companies code that fails them is blocked from shipping. Without tests, every change to a shared codebase is a gamble that you remembered to re-check everything by hand — nobody remembers, which is why untested codebases rot into “do not touch that file” territory. This course teaches you to write those checks, and, in its second half, to debug with a method when they go red.

When a test fails

Change add above to return a - b and run again. The first assert becomes assert -1 == 5, which is false, so Python raises AssertionError and the print never happens.

That crash is the test doing its job. A failing test is not an annoyance, it is information: this exact fact about my code is no longer true. You found the bug the moment you created it, at your desk, instead of a user finding it next week.

Two words you will use constantly:

• a test passes (or is green) when its assertions are all true
• a test fails (or is red) when an assertion is false or the code crashes