Returning same type as self for arithmetic in subclasses

Tim Peters tim.peters at
Fri Jan 7 20:09:27 EST 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>.

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