[Python-Dev] subclassing builtin data structures

[Ethan Furman]

On 02/13/2015 02:31 PM, Serhiy Storchaka wrote:
> On 13.02.15 05:41, Ethan Furman wrote:
>> So there are basically two choices:
>>
>> 1) always use the type of the most-base class when creating new instances
>>
>>     pros:
>>       - easy
>>       - speedy code
>>       - no possible tracebacks on new object instantiation
>>
>>     cons:
>>       - a subclass that needs/wants to maintain itself must override all
>>         methods that create new instances, even if the only change is to
>>         the type of object returned
>>
>> 2) always use the type of self when creating new instances
>>
>>     pros:
>>       - subclasses automatically maintain type
>>       - much less code in the simple cases [1]
>>
>>     cons:
>>       - if constructor signatures change, must override all methods which
>>         create new objects
> 
> And switching to (2) would break existing code which uses subclasses with constructors with different signature (e.g.
> defaultdict).

I don't think defaultdict is a good example -- I don't see any methods on it that return a new dict, default or
otherwise. So if this change happened, defaultdict would have to have its own __add__ and not rely on dict's __add__.


> The third choice is to use different specially designed constructor.
> 
> class A(int):
> 
> --> class A(int):
> ...     def __add__(self, other):
> ...         return self.__make_me__(int(self) + int(other))
> 
> ...     def __repr__(self):
> ...         return 'A(%d)' % self

How would this help in the case of defaultdict?  __make_me__ is a class method, but it needs instance info to properly
create a new dict with the same default factory.

--
~Ethan~

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