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On Thu, Feb 12, 2015 at 4:36 PM, Ethan Furman <ethan@stoneleaf.us> wrote:

I suspect the last big hurdle to making built-in data structures nicely subclassable is the insistence of such types to
return new instances as the base class instead of the derived class.

In case that wasn't clear ;)

\--> class MyInt(int):
... def \_\_repr\_\_(self):
... return 'MyInt(%d)' % self
...
\--> m = MyInt(42)
\--> m
MyInt(42)
\--> m + 1
43
\--> type(m+1)

Besides the work it would take to rectify this, I imagine the biggest hurdle would be the performance hit in always
looking up the type of self. Has anyone done any preliminary benchmarking? Are there other concerns?

Actually, the problem is that the base class (e.g. int) doesn't know how to construct an instance of the subclass -- there is no reason (in general) why the signature of a subclass constructor should match the base class constructor, and it often doesn't.

So this is pretty much a no-go. It's not unique to Python -- it's a basic issue with OO.

--Guido van Rossum (python.org/\~guido)