First of all,
print shouldn't be capitalized (I'm guessing that's a typo in your question, not in your code).
In 3.0 the event API has changed a bit (and hasn't yet been documented fully). An InputEvent is an object, and you check its type to see what it is. Depending on its type, you can access its members. For example:
if event is InputEventMouseButton:
if event.button_index == BUTTON_WHEEL_UP:
You can also use
get_button_index() like in your question, but you would need the
() because it's a function.