Sorry about that. You can work around that somewhat by making the attribute rw, but with an internal writer, say _set_visitors(), so you can set the visitors attribute from within your code, but visitors() can't be used to set it.

It's not perfect, but the only way around it that I can see would be to dynamically make the attribute rw, set the value and then make it ro, which isn't pretty. I'm assuming that can be done, though I've never tried.