Exceptions: try, except, raise

You will be able to

• ✓ Separate syntax errors (never run) from runtime errors (can be caught)
• ✓ Use guards and status strings here; try/except/raise on a laptop
• ✓ Refuse garbage with a named failure — do not store “n/a” as a phone number

A form that sends “twelve” is expected

People type twelve, 12, and 12.0. A night job receives an empty file. Syntax errors (a missing colon) never start the program. Runtime errors happen while it runs: int("twelve") on a laptop raises ValueError. try/except catches the error you expect. raise is how you refuse garbage with a name so callers cannot ignore it. User-defined errors are classes on a laptop; here a status string — OK or Reject — starts the same policy.

Guard clauses are the sandbox version: if len(pin) != 6: return "Reject". On a laptop you might raise ValueError("PIN must be 6 digits"). Either way, name the policy. A bare except that swallows everything will hide a bug in your own arithmetic — do not do that in production.

def parse_seats(text):
    try:
        n = int(text)
    except ValueError:
        raise ValueError("seats must be whole digits") from None
    if n < 0:
        raise ValueError("seats cannot be negative")
    return n

Fail at the edge, keep the core boring

late_fee(days) can assume days is already an int >= 0. parse_days(text) is where “twelve” dies. Split those jobs so tests for the fee table never need a try. Exceptions are policy for the edge and for truly unexpected disk failures — not a second if/else language for every branch.

raise in a library is a contract: “I will not return a fake 0 for a bad GSTIN.” Returning 0 looks like a free bill. Reject is louder.

Words that matter

Guard

An early check that refuses bad input.

Exception

A named failure the machine (or you) raises — try/except on a laptop.

raise

Laptop: throw a named error so callers must handle it.

ValueError

“This value cannot be used” — usual choice for bad form text.

Common mistakes

On a full Python install — try / except / raise

Catch the conversion you expect. raise a named error for policy. Never except: pass around the whole payroll.

stdlib — no pip

Named failure at the edge.

def parse_pin(text):
    if len(text) != 6:
        raise ValueError("PIN must be 6 characters")
    return text

try:
    print(parse_pin("411"))
except ValueError as err:
    print("Reject", err)

Example program — Seats cannot be negative

A human message, not a crash dump. Core maths never sees -3.

Your turn — Reject 12

code = "12". Print Reject when len is not 6.

Self-assessment

Check your understanding before you mark the lesson complete.