A habit I've gotten into is to single or double quote anything I am templating into a string, incase, by some bug, the value is empty. For instance if `self.default_installer_name` is empty the message would read: `Default installer  does not appear in configured...`, if you were to quote it, then it would be a little more obvious that there was a problem: `Default installer '' does not appear in configured...`

This is not a hard and fast rule, and I'll leave it up to your preference, but it is just a suggestion.