But on other hand, the error depends on the value of format specifier:

>>> format('abc', '')
'abc'
>>> format('abc', 'j')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Unknown format code 'j' for object of type 'str'
>>> format([1, 2, 3], '')
'[1, 2, 3]'
>>> format([1, 2, 3], 'j')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to list.__format__

If keep TypeError I think it would be better to restore former error message "non-empty format string".