try/except — the seatbelt AI forgets to put on

Every non-trivial script has lines that might fail. Reading a file that doesn't exist. Parsing a string the user mistyped. Calling an API that returns a 500 instead of a 200. Indexing a list that turned out empty.

When the line that fails has no protection around it, Python halts the whole program and prints a traceback. Sometimes that's what you want — a loud crash beats silent corruption. But for expected failure modes — "the user might paste something that isn't a number," "the API might be down for ten seconds" — you want the script to keep running and recover. That's what try/except is for.

The mental model

Read this two-line shape out loud:

try:
    result = int("42")
except ValueError:
    print("not a number")

In English: "Try to run the indented code. If a ValueError happens anywhere inside the try block, jump down to the except block instead of crashing. Otherwise, skip the except block entirely."

Two paths through the code. Python picks one based on whether the risky line raised the exception you named:

• Happy path: the try body runs all the way through. The except block is skipped. Execution continues past both blocks.
• Sad path: somewhere inside the try body, a ValueError gets raised. Python immediately abandons the rest of the try body, jumps into the except block, runs that, and continues past both blocks.

Whichever way it went, the program does not crash and the next line of your script gets to run.

A worked example

The editor on the right has the canonical first shape:

try:
    result = int("42")
    print(result)
except ValueError:
    print("not a number")

Run it. "42" is a valid integer string, so int("42") succeeds, returns 42, and the print fires. The except block never runs. Output:

42

Now imagine the input was "forty-two" instead. int("forty-two") would raise ValueError, the second print inside try would be skipped, and the except block would print not a number. Same code, two outcomes — controlled by the input.

Where AI specifically gets this wrong

Cursor writes the happy path almost every time and leaves you to add the seatbelt. Look at the typical AI snippet for "fetch a user's score from the API":

data = response.json()
score = data["user"]["score"]
print(score)

There are at least four things that can blow up: response.json() if the body isn't JSON, data["user"] if there's no user key, data["user"] ["score"] if the user has no score, and the print if score is some weird object. AI assumes none of those happen.

Your job, reading AI code, is to spot the lines that can fail and decide which deserve a try/except wrapper. This chapter teaches you the moves to do that fluently. Hit Run. The string "42" parses fine, so the except block never fires. We'll break it on purpose two steps from now.