An optional change that I may make later if you don't want to handle here is to just make a list. Something like:

```python
link_classes = [link_class]
if 'persona-button' not in link_classes:
    link_classes.append('persona-button')
if color:
    link_classes.append(color)
# etc. etc.
return browserid_button(text, next, ' '.join(link_classes), attrs, fallback_href)
```