I used to click through the app and watch it, and that told me nothing the next Tuesday when the code changed. A test is a sentence about the program that the computer can re-check forever.
About 2 min of reading
Write one claim about a tiny function and see the computer confirm it.
Define a function that adds two numbers. This is the program under test.
Before you run it, say out loud what it will print. Then open the answer.
Call it and look at the answer. You can see 5, but that is only you seeing it once.
Before you run it, say out loud what it will print. Then open the answer.
Now make the computer check it. The assert passes, so nothing prints and you get a clean prompt.
Before you run it, say out loud what it will print. Then open the answer.
Make a false claim. The assert raises and names the line, which is the report.
Before you run it, say out loud what it will print. Then open the answer.
You could read the code and reason about it. For a week that works. Then the code changes, or someone else changes it, and your reasoning is stale. A test moves the truth from your head into the machine. It is a claim you can run today, next week, and after a stranger edits the file.
A demo shows the program working once. A test states what must always be true and lets the computer confirm it on every run. The difference is the word always. A demo is a photograph, a test is a rule the photograph is checked against.
assert add(2, 3) == 5Read it as a promise: adding 2 and 3 must give 5. The computer runs it and either stays silent or raises. That silence is the whole meaning of a passing test.
assert add(2, 3) == 5
assert add(-1, 1) == 0You write the sentence once and the computer runs it every time you run the file. It does not forget, it does not get bored, it does not need you to click. That is what makes it different from watching.
assert add(2, 3) == 6This one raises. That raise is not a mystery, it is a report: the program says 5 and you claimed 6. A red test is a machine-written note about a real mismatch.