Problem of understanding inheritance

[Robert Kern]

matilda matilda wrote:
> 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?

You did not override __add__(). I'm almost certain that the implementation of
datetime.date.__add__ creates a new datetime.date object explicitly rather than
using self.__class__ to figure out what it should construct.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco