This is sort of OK for a command-line utility, but it would be better to instead have this propagate up an exception. If this might ever be invoked elsewhere, it'll be really frustrating to have your process just exit instead of getting an exception. I'd have it throw an exception of your own.

In particular, I'd create an exception hierarchy:

```
class NbackException(Exception):
    pass

class MySpecificException(NbackException):
    pass
```
That way, people invoking the code could catch `NbackException`s of all types if they were so inclined.

Also, FWIW, if you're exiting a process because of an error, it should exit with a non-zero status code, like `exit(1)`. Also, customarily, different exit statuses should map to different modes of failure.