Returning same type as self for arithmetic in subclasses

[Max M]

Tim Peters wrote:

> Yes, and all builtin Python types work that way.  For example,
> int.__add__ or float.__add__ applied to a subclass of int or float
> will return an int or float; similarly for a subclass of str.  This
> was Guido's decision...

I will not discuss it with him. He is usually right :-s


> Generally speaking, no.  But I'm sure someone will torture you with a
> framework that purports to make it easy <wink>.

Apparently not... But here is my solution.

If anybody is interrested. It should also be obvious what I am working on.

Btw. I really love doctests ... Unittests are a nice idea. But doctest 
is a really practical solution.

###############################

class vDatetime(datetime):
     """
     A subclass of datetime, that renders itself in the iCalendar datetime
     format.

     >>> dt = vDatetime(1970, 1,1, 12, 30, 0)
     >>> str(dt)
     '19700101T123000'

     >>> dt2 = vDatetime(1970, 1,1, 0, 0, 0)
     >>> str(dt - dt2)
     'PT12H30M'

     Adding is not allowed
     >>> dt + dt2
     Traceback (most recent call last):
         ...
     AttributeError: 'NotImplementedType' object has no attribute 'days'
     """

     def __init__(self, *args, **kwargs):
         datetime.__init__(self, *args, **kwargs)
         self.params = Params()

     def __add__(self, other):
         return self._to_vdatetime(datetime.__add__(self, other))

     def __sub__(self, other):
         return self._to_vdatetime(datetime.__sub__(self, other))

     def _to_vdatetime(self, result):
         if hasattr(result, 'timetuple'):
             return vDatetime(*result.timetuple()[:6])
         return vDuration(result.days, result.seconds)

     def fromstring(st):
         "Class method that parses"
         try:
             timetuple = map(int, ((
                 st[:4],     # year
                 st[4:6],    # month
                 st[6:8],    # day
                 st[9:11],    # hour
                 st[11:13],    # minute
                 st[13:15],    # second
                 )))
         except:
             raise ValueError, 'Wrong format'
         return vDatetime(*timetuple)
     fromstring = staticmethod(fromstring)

     def __str__(self):
         return self.strftime("%Y%m%dT%H%M%S")



-- 

hilsen/regards Max M, Denmark

http://www.mxm.dk/
IT's Mad Science