Vibe coding and AI agents classes in Germany

# The kind of answer an AI assistant often gives (written by us to show a typical mistake)
def valid_postcode_ai(code):
    try:
        n = int(code)
    except ValueError:
        return False
    return 10000 <= n <= 99999

# Our tests, written BEFORE reading the code: input -> the answer we expect
tests = {
    "10115": True, "80331": True, "01067": True,
    "1234": False, "123456": False, "12a45": False,
    " 10115": False, "10_115": False,
}

def check(fn):
    failed = [code for code, want in tests.items() if fn(code) != want]
    print(fn.__name__, "passed", len(tests) - len(failed), "of", len(tests), "| failed:", failed)

check(valid_postcode_ai)

Three failures, each with a different cause. A code that starts with 0, such as 01067, becomes the number 1067, which is below 10000, so a perfectly valid five-digit code is refused. A code with a space in front is accepted, because Python's int quietly ignores the space. And 10_115 is accepted too, because Python allows underscores between digits in numbers. None of these show up if you test the function with one ordinary postcode, which is what most people do.

def valid_postcode(code):
    # exactly five characters, every one of them an ASCII digit 0 to 9
    return len(code) == 5 and code.isascii() and code.isdigit()

check(valid_postcode)

Why does the fixed version also check isascii? Because Python's isdigit accepts digits from other writing systems too, and a German postcode field should not. A learner who asks an AI "is this correct?" about Version 1 may well be told yes. A learner who wrote the eight tests first knows the answer in a second. With agents the lesson scales up: an agent can change twenty files in a minute, and the tests are what tell you whether you can keep them.

1. Write tests first. Pick a small task, a phone number, a date or an email, and write ten tests before asking any AI for code.
2. Ask and check. Get an answer from an AI tool you are allowed to use and run your tests against it. Count the failures.
3. Break the fix. Find an input that Version 2 handles in a way you think is wrong, and argue for your view.
4. Adults with agents. Give a coding agent the tests and the task together, and review its change against the tests before accepting it.

The Einheitenzeitgesetz puts Germany one hour ahead of UTC (MEZ) through the winter and two hours ahead (MESZ) through the summer. Winter time began on 25 October 2026; summer time returns on 28 March 2027. India keeps UTC+5:30 throughout. Example rows only; no group time is promised.

Trial slots are offered in whatever time zone your device uses. Adults working full time often choose one to one, so that a lesson can move when a meeting runs late.