Control flow · Lesson 6 of 18 · 16 min

Exceptions

Errors as values you can catch, raise, and design around.

Catch specific exceptions
Use else/finally correctly
Raise with intent, not as flow for the happy path

EAFP, not LBYL

Python style prefers Easier to Ask Forgiveness than Permission: try the operation, catch the specific error. Looking before you leap (`if key in d`) races in concurrent code and often duplicates work the operation will do anyway.

Always catch the narrowest type you can handle. `except Exception` is a last resort. Bare `except:` also catches `KeyboardInterrupt` and `SystemExit` — almost never what you want.

try / except / else / finally

def parse_int(text):
    try:
        value = int(text)
    except ValueError as exc:
        return f"bad: {exc}"
    else:
        return f"ok: {value}"
    finally:
        pass

print(parse_int("42"))
print(parse_int("nope"))

The clauses have jobs

`except` handles failure. `else` runs only if no exception was raised in `try` — a good place for code that should not catch its own errors. `finally` always runs, whether you return, raise, or succeed, which is why it is the right home for cleanup when a context manager is not available.

`raise` without an argument inside an except block re-raises the active exception. `raise NewError('msg') from exc` chains exceptions so the traceback shows both the cause and the new meaning you attached.

Raising and chaining

class ConfigError(Exception):
    pass

def load_port(raw):
    try:
        return int(raw)
    except ValueError as exc:
        raise ConfigError(f"port must be an int, got {raw!r}") from exc

try:
    load_port("abc")
except ConfigError as exc:
    print(type(exc).__name__, exc)
    print("cause:", type(exc.__cause__).__name__)

Check yourself

Quiz

1.The else clause of try runs when:
2.Why is `except:` (bare) a problem?
3.`raise New from exc` stores the original on:

Practice

Exercise

Write `safe_div(a, b)` that returns a / b as a float. If b is 0, return None instead of raising. Any other error should still raise.

Tutor

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