Errors and exceptions

Use exceptions when something is wrong and the current path should stop. Raise an exception with raise, and handle it with try and except.

def parse_port(text: str) -> int:
    port = int(text)
    if port < 0 or port > 65535:
        raise ValueError("port must be between 0 and 65535")
    return port

actor main(env):
    for text in ("8080", "70000"):
        try:
            port = parse_port(text)
        except ValueError as e:
            print("invalid input:", e)
        else:
            print("port:", port)

    env.exit(0)

try structure

A try statement can contain these parts:

• try for the code that may fail
• except for handling specific exception types
• else for code that should run only when nothing failed
• finally for cleanup that should happen either way

try:
    value = parse_port("9000")
except ValueError as e:
    print("bad input:", e)
else:
    print("ready to use:", value)
finally:
    print("done")

except runs only for matching exceptions. else runs only when the try block completed without raising. finally runs whether the try block succeeded or failed.