Exit in Python

Python has several ways to make a program exit.

exit should probably only be used to exit a Python REPL because it’s not safe to assume it’s defined.

The sys.exit function is safe, but you might prefer to raise SystemExit instead because:

1. they do the same thing and have the same interface
2. sys.exit requires an additional import (import sys)
3. sys.exit raises SystemExit without being explicit about the fact it raises an exception

You can exit with a status code, such as raise SystemExit(0) or raise SystemExit(1). If you want to print an error message to stderr and then exit with status code 1, you can use raise SystemExit("your error message here").

The SystemExit exception is derived from BaseException, not Exception, so the common try: ...; except Exception: ... will not catch it (which is good).

Exiting by raising an exception is important for many reasons including making code easier to test. However, there are rare cases where a program should be exited immediately without raising an exception. In those cases, you can use the os._exit function.

CLI parsing libraries

CLI parsing libraries often have their own functions or exceptions for exiting. Some of them print extra info such as the CLI’s usage message.

• argparse’s exit functions
• click’s exit exceptions

click.ClickException

raise click.ClickException("your error message here")

result:

Error: your error message here

click.BadOptionUsage

raise click.BadOptionUsage("option_name_here", "your error message here")

result:

Usage: deploy [OPTIONS]
Try 'deploy --help' for help.

Error: your error message here

click.BadArgumentUsage

raise click.BadArgumentUsage("your error message here")

result:

Usage: deploy [OPTIONS]
Try 'deploy --help' for help.

Error: your error message here

Click has a few other exception types that you probably won’t use directly.