I left a comment on Rietveld, but repeating it here because for me those messages often go to spam:

I think you want to make this more general, by modifying the message for the TypeError that follows. Any type that doesn't provide it's own __format__ should produce an error when using a non-empty format string.

For example:

>>> format({}, 'a')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__

Would be better as:
TypeError: non-empty format string passed to __format__ for object of type "class <'dict'>"

(Or some prettier way to print out the type).