Problem of understanding inheritance

[matilda matilda]

Hi all,

I searched for a while, but didn't found answer to my question.

I wrote the following little program:
====================================
#!/usr/bin/python
import datetime as dt
class MyClass(dt.date):
    def __init__(self, *args, **kwargs):
        super(MyClass, self).__init__(*args, **kwargs)

    def addday(self, day=0):
        my = self + dt.timedelta(day)
        return my

    def getfirstofmonth(self):
        return self.replace(day=1)


if __name__ == '__main__':
    my = MyClass(2006,10,1)
    print "Type before", type(my)
    my = my.getfirstofmonth()
    print "Type after", type(my)
    my = my.addday(2)
    print "Type after add", type(my)
====================================

What I don't understand is, why the type of the returning object
'my' changes by executing my.addday(2).
Why does addday return the datetime.date-object and not 
an object of class MyClass?
What am I missing?

Can anybody clearify?

Thank you in advance.
Best regards
Andreas