Format strings aren't a template language (Python even has Template Strings for that purpose), and being able to do this is quite useful in eg. debug logs it's

Format strings aren't a template language (Python even has Template Strings for that purpose), and being able to do this is quite useful in eg. debug logs it's a shorter and easier to write "{foo.x} ({foo.__class__.__name__})".format(foo=something). Conversely they don't prohibit expensive properties either (eg. ORM-based lookups), or generating misleading output or whatever.