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Terminating

There are some cases where you might want to terminate a command at some point, and stop all subsequent execution.

It could be that your code determined that the program completed successfully, or it could be an operation aborted.

Exit a CLI program

You can normally just let the code of your CLI program finish its execution, but in some scenarios, you might want to terminate at some point in the middle of it. And prevent any subsequent code to run.

This doesn't have to mean that there's an error, just that nothing else needs to be executed.

In that case, you can raise a typer.Exit() exception:

import typer

existing_usernames = ["rick", "morty"]


def maybe_create_user(username: str):
    if username in existing_usernames:
        typer.echo("The user already exists")
        raise typer.Exit()
    else:
        typer.echo(f"User created: {username}")


def send_new_user_notification(username: str):
    # Somehow send a notification here for the new user, maybe an email
    typer.echo(f"Notification sent for new user: {username}")


def main(username: str):
    maybe_create_user(username=username)
    send_new_user_notification(username=username)


if __name__ == "__main__":
    typer.run(main)

There are several things to see in this example.

  • The CLI program is the function main(), not the others. This is the one that takes a CLI argument.
  • The function maybe_create_user() can terminate the program by raising typer.Exit().
  • If the program is terminated by maybe_create_user() then send_new_user_notification() will never execute inside of main().

Check it:

$ python main.py Camila

User created: Camila
Notification sent for new user: Camila

// Try with an existing user
$ python main.py rick

The user already exists

// Notice that the notification code was never run, the second message is not printed

Tip

Even though you are rasing an exception, it doesn't necessarily mean there's an error.

This is done with an exception because it works as an "error" and stops all execution.

But then Typer (actually Click) catches it and just terminates the program normally.

Exit with an error

typer.Exit() takes an optional code parameter. By default, code is 0, meaning there was no error.

You can pass a code with a number other than 0 to tell the terminal that there was an error in the execution of the program:

import typer


def main(username: str):
    if username == "root":
        typer.echo("The root user is reserved")
        raise typer.Exit(code=1)
    typer.echo(f"New user created: {username}")


if __name__ == "__main__":
    typer.run(main)

Check it:

$ python main.py Camila

New user created: Camila

// Print the result code of the last program executed
$ echo $?

0

// Now make it exit with an error
$ python main.py root

The root user is reserved

// Print the result code of the last program executed
$ echo $?

1

// 1 means there was an error, 0 means no errors.

Tip

The error code might be used by other programs (for example a Bash script) that execute your CLI program.

Abort

There's a special exception that you can use to "abort" a program.

It works more or less the same as typer.Exit() but will print "Aborted!" to the screen and can be useful in certain cases later to make it explicit that the execution was aborted:

import typer


def main(username: str):
    if username == "root":
        typer.echo("The root user is reserved")
        raise typer.Abort()
    typer.echo(f"New user created: {username}")


if __name__ == "__main__":
    typer.run(main)

Check it:

$ python main.py Camila

New user created: Camila

// Now make it exit with an error
$ python main.py root

The root user is reserved
Aborted!