On Thu, Feb 12, 2015 at 4:36 PM, Ethan Furman <et...@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. -- --Guido van Rossum (python.org/~guido)
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