Exceptions and resource boundaries

The idea

Exceptions transfer control from a failing operation to a matching handler. Catch only failures you can meaningfully handle. Converting text to an integer may raise ValueError, while a programming mistake elsewhere should usually remain visible. Keep the try block narrow so an unrelated error is not mistaken for bad input. Domain validation still follows successful parsing: an integer can be syntactically valid but outside the accepted range.

Resources such as files need cleanup even when a later operation fails. A with statement delegates acquisition and release to a context manager, allowing cleanup on both ordinary completion and exceptions. Do not suppress failures merely to make a program appear successful. Prefer a documented return value for expected absence and a clear exception for a violated contract.

def nonnegative_count(text):
    try:
        value = int(text)
    except ValueError as error:
        raise ValueError("Count must be an integer") from error
    if value < 0:
        raise ValueError("Count must be nonnegative")
    return value

print(nonnegative_count("12"))

12

Conversion and range validation establish different parts of the contract. Chaining the conversion error preserves diagnostic context without changing successful results.