Exceptions and domain boundaries

Opening your lesson

An exception interrupts normal control flow and carries information about a failure. Catch an exception where you can make a meaningful decision, and name the narrow type you expect. Catching every exception can turn a programming mistake into a misleading fallback. A domain exception can describe a failed business rule without exposing how the rule was implemented. The else block runs when the try block succeeds; finally runs as control leaves either path. You'll parse request values, validate a booking rule, preserve error categories, and record cleanup. Every failure is local and reproducible. No task contacts an external system or requires a real payment, account, or patient record.

def parse_retries(tokens):
    result = []
    for token in tokens:
        try:
            result.append(int(token))
        except ValueError:
            result.append(-1)
    return result

try: value = int(text) can catch ValueError for malformed integer text. An empty string and "2.5" both fail integer conversion. Catching that type leaves unrelated programming errors visible.

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