Python exceptions: translate an input error without hiding its cause

Operation contract

The receipt parser catches only the ValueError raised by integer conversion, then raises a domain error with explicit chaining. Negative amounts fail a separate business rule. The caller distinguishes parsing rejection from successful conversion instead of catching every Exception and manufacturing a zero amount.

Failure and ownership boundary

A broad catch can conceal programming errors such as a missing attribute. Finally also runs during failure, so returning from finally can suppress an exception the caller needed to see. Keep cleanup in Python context managers: clean up on success and failure and keep error messages bounded rather than echoing an entire sensitive import record.

Working program

def parse_minor_amount(text):
    try:
        amount = int(text)
    except ValueError as cause:
        raise ValueError("integer amount required") from cause
    if amount <= 0:
        raise ValueError("positive amount required")
    return amount

print(parse_minor_amount("125"))
try:
    parse_minor_amount("12x")
except ValueError as failure:
    print(str(failure))
    print(type(failure.__cause__).__name__)

Output

125
integer amount required
ValueError

Costs and limits

Conversion cost grows with input length and retained integer digits. This fixture uses tiny strings; an external request must cap length before converting.

Common Mistakes

• Catch the failure you can handle.
• Do not return from finally to hide a pending error.

Connected lessons

Python context managers: clean up on success and failure, Python input exercise: accept an explicit integer grammar, Java exceptions: recovery boundaries and failure context.

Follow the related contract

Python ExceptionGroup: handle selected failures and retain the rest, Python exceptions reference: raise, propagate, chain and clean up, Python exception interview: chained causes and a finally return that hides failure.

Follow the service contract

Python assert versus input validation: optimized execution changes the contract.

Check this related boundary

Python interview: a return from finally replaces a pending exception.

Trace the related workflow

Python interview: try else runs only after the try body succeeds.