def DynamicDocstringMeta(reps):
    def __new__(cls, name, bases, attrs):
        attrs['__doc__'] = attrs['__doc__'] % reps
        return type.__new__(cls, name, bases, attrs)
    return type('', (type, ), {'__new__': __new__})

# test scenario

class Test(object):
    """This docstring can change! (%s, %d)"""
    __metaclass__ = DynamicDocstringMeta(('test', 3))

>>> Test().__doc__
'This docstring can change! (test, 3)'