Python exceptions reference: raise, propagate, chain and clean up

Operation contract

The bounded parser translates an integer-conversion ValueError into a domain ValueError with an explicit cause. A finally suite appends a cleanup marker regardless of acceptance or rejection. The caller catches only the declared domain failure. Two calls show cleanup on both paths without a finally return that would hide the rejected input.

Failure and ownership boundary

Exception messages can reveal received values if they are copied into logs; this fixture reports a fixed domain message. Bare except and broad BaseException handling can interfere with process control. Resource acquisition itself may fail before a finally-managed body starts, which is why Python ExitStack: unwind partially acquired resources has its own pattern. Python ExceptionGroup: handle selected failures and retain the rest needs except* rather than flattening all failures.

Working program

cleanup = []
def parsed_quantity(text):
    try:
        try:
            return int(text)
        except ValueError as cause:
            raise ValueError("quantity conversion rejected") from cause
    finally:
        cleanup.append("released")

print(parsed_quantity("3"))
try:
    parsed_quantity("invalid")
except ValueError as failure:
    print(str(failure))
    print("cause retained:", failure.__cause__ is not None)
print(cleanup)

Output

3
quantity conversion rejected
cause retained: True
['released', 'released']

Costs and limits

The fixture has fixed small input. Int conversion can accept more syntax than a field grammar and allocate larger integers for longer text; see the strict parser rather than treating this reference operation as complete wire validation.

Common Mistakes

• Cleanup should not replace the pending exception with a return.
• Catch the intended failure category and keep received secrets out of messages.

Connected lessons

Python type conversion: parsing success is not field validity, Python exception interview: chained causes and a finally return that hides failure, Python ExitStack: unwind partially acquired resources.