Sorry, as with all similar subclasses, class op subclass = class unless one explicitly subclasses the answer or overrides the __op__ method to do that for you.

class octint(int):
    'int that displays as octal'
    def __str__(self):
        return oct(self)
    __repr__ = __str__

mode = octint(0o640)
print(mode + 4, octint(mode+4))

def __add__(self, other):
    return octint(int.__add__(self, other))
    # octint(self+other) is infinite recursion 

octint.__add__ = __add__
print(mode+4)
>>> 
420 0o644
0o644