Alexander Belopolsky <belopol...@users.sourceforge.net> added the comment:
I would like to take another shot at this. The link in Amaury's closing comment no longer works, so here is the relevant post: """ Returning same type as self for arithmetic in subclasses Tim Peters tim.peters at gmail.com Sat Jan 8 02:09:27 CET 2005 [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/pipermail/python-list/2005-January/925838.html As I explained in my previous post, the same argument, "base class has no idea what requirements may exist for invoking a subclass's constructor", applies to class methods, but they nevertheless consistently construct subclass instances: >>> class d(datetime): pass >>> d.utcfromtimestamp(0) d(1970, 1, 1, 0, 0) >>> d.fromtimestamp(0) d(1969, 12, 31, 19, 0) >>> d.combine(date(1,1,1), time(1,1)) d(1, 1, 1, 1, 1) Similar example for the date class: >>> class Date(date): pass >>> Date.fromordinal(1) Date(1, 1, 1) In my view it is hard to justify that for a Date instance d, and integer days, Date.fromordinal(d.toordinal() + days) happily produces a Date instance, but d + timedelta(days) returns a basic date instance. ---------- assignee: -> belopolsky components: +Extension Modules -Library (Lib) nosy: +mark.dickinson resolution: wont fix -> status: closed -> open versions: +Python 3.2 -Python 2.5 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue2267> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
