>>> class Foo(object):
...     pass
... 
>>> "asdads" % Foo()
'asdads'


Doesn't look like it's supposed to be this way.

As per the documentation:

"If format requires a single argument, values may be a single non-tuple object. [5] Otherwise, values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary)."

Tested and confirmed on 2.5, 2.6, 2.7 (both old and new style classes) and 3.2.