[Python] If I subclass int, how to make arithmetic methods return the derived type?

I want to construct objects which support arithmetic operations and have extra methods, e.g:

class Timestamp(int):
def as_seconds(self):
return self / 90000


The extra method works:

>>> ts1 = Timestamp(135000)
>>> ts1
135000
>>> ts1.as_seconds()
1.5


Arithmetic operations work, but they return instances of the base type instead of the derived type, so I cannot use the extra method anymore:

>>> ts2 = ts1 + 22500
>>> ts2
157500
>>> ts2.as_seconds()
Traceback (most recent call last):
File "<python-input-53>", line 1, in <module>
ts2.as_seconds()
^^^^^^^^^^^^^^
AttributeError: 'int' object has no attribute 'as_seconds'


Is there a simple way to make that work without having to wrap all arithmetic methods? (i.e. __add__, __sub__, __mul__, etc...)

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