Not Invented Here syndrome, at all? Sure, I understand wanting to use Python for templates rather than relying on an external application. But why ignore the many, many existing template libraries? Some of those libraries have solved problems which you don't realise you have yet.

Case in point, are you planning to support this 'dead simple' template language for users of your hosting site? If so, what happens if I try to include something outside the site root, in a Dropbox area which I control? https://github.com/braceio/tags/blob/master/tags/tags.py#L13