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

Ethan Furman ethan at stoneleaf.us
Sat Feb 14 02:12:42 CET 2015

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.makeme(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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