[Python-Dev] subclassing builtin data structures (original) (raw)

MRAB python at mrabarnett.plus.com
Fri Feb 13 02:46:21 CET 2015

On 2015-02-13 00:55, 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.

Really?

class BaseInt: ... def init(self, value): ... self._value = value ... def add(self, other): ... return type(self)(self._value + other) ... def repr(self): ... return '%s(%s)' % (type(self), self._value) ... class MyInt(BaseInt): ... pass ...

m = BaseInt(42) m <class '__main__.BaseInt'>(42) m + 1 <class '__main__.BaseInt'>(43) type(m + 1) <class '__main__.BaseInt'>

m = MyInt(42) m <class '__main__.MyInt'>(42) m + 1 <class '__main__.MyInt'>(43) type(m + 1) <class '__main__.MyInt'>

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