[Max M] > """ > I subclass datetime and timedelta > > >>> dt = myDatetime(1970,1,1) > >>> type(dt) > <class 'dtime.myDatetime'> > > >>> td = myTimedelta(hours=1) > >>> type(td) > <class 'dtime.myTimedelta'> > > But when I do arithmetic with these classes, they return datetime and > timedelta, ... > >>> new_time = dt + td > >>> new_time > datetime.datetime(1970, 1, 1, 1, 0) > > >>> type(new_time) > <type 'datetime.datetime'>
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, based on that an implementation of any method in a base class has no idea what requirements may exist for invoking a subclass's constructor. For example, a subclass may restrict the values of constructor arguments, or require more arguments than a base class constructor; it may permute the order of positional arguments in the base class constructor; it may even be "a feature" that a subclass constructor gives a different meaning to an argument it shares with the base class constructor. Since there isn't a way to guess, Python does a safe thing instead. > where I want them to return myDatetime and myTimedelta > > So I wondered if there was a simlpler way to coerce the result into my > desired types rather than overwriting the __add__, __sub__ etc. methods? Generally speaking, no. But I'm sure someone will torture you with a framework that purports to make it easy <wink>. -- http://mail.python.org/mailman/listinfo/python-list
