[Python-Dev] subclassing builtin data structures

[Ethan Furman]

On 02/12/2015 05:46 PM, MRAB wrote:
> 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?

What I was asking about, and Guido responded to, was not having to specifically override __add__, __mul__, __sub__, and
all the others; if we do override them then there is no problem.

--
~Ethan~

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