[Python-Dev] subclassing builtin data structures

[Ethan Furman]

On 02/12/2015 04:55 PM, Guido van Rossum wrote:
> On Thu, Feb 12, 2015 at 4:36 PM, Ethan Furman <ethan at stoneleaf.us <mailto:ethan at 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)
>     <class 'int'>
> 
>     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.

Thank you.

--
~Ethan~

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