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>>> 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.