[Python-Dev] subclassing builtin data structures

[Steven D'Aprano]

On Sat, Feb 14, 2015 at 01:26:36PM -0500, Alexander Belopolsky wrote:
> On Sat, Feb 14, 2015 at 7:23 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> 
> > Why can't int, str, list, tuple etc. be more like datetime?
> 
> 
> They are.  In all these types, class methods call subclass constructors but
> instance methods don't.

But in datetime, instance methods *do*.

Sorry that my example with .today() was misleading.

py> from datetime import datetime
py> class MyDatetime(datetime):
...     pass
...
py> MyDatetime.today()
MyDatetime(2015, 2, 15, 12, 45, 38, 429269)
py> MyDatetime.today().replace(day=20)
MyDatetime(2015, 2, 20, 12, 45, 53, 405889)


> In the case of int, there is a good reason for this behavior - bool.  In
> python, we want True + True == 2.

Sure. But bool is only one subclass. I expect that it should be bool's 
responsibility to override __add__ etc. to return an instance of the 
parent class (int) rather have nearly all subclasses have to override 
__add__ etc. to return instances of themselves.



-- 
Steve