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

Ionel Cristian Mărieș contact at ionelmc.ro
Fri Feb 13 16:58:36 CET 2015

Can we at least make it use the constructor (if there's a custom one)? Seems like a reasonable compromise to me (let whoever implements a custom new deal with argument variance).

Eg, make it use a new like this:

class FancyInt(int): ... def new(self, value): ... return int.new(FancyInt, value) ... ... def repr(self): ... return "FancyInt(%s)" % super().repr() ... x = FancyInt(1)

x FancyInt(1) x += 1 x # it should be FancyInt(2) 2

Thanks, -- Ionel Cristian Mărieș, blog.ionelmc.ro

On Fri, Feb 13, 2015 at 6:01 AM, Guido van Rossum <guido at python.org> wrote:

On Thu, Feb 12, 2015 at 7:41 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

On 02/12/2015 06:39 PM, Alexander Belopolsky wrote:

> In my view, a constructor is no different from any other method. If the designers of the subclass decided to change the > signature in an incompatible way, they should either override all methods that create new objects or live with tracebacks. > On the other hand, if all I want in my Date class is a better format method, I am forced to override all operators > or have my objects silently degrade [...] 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 Unless there are powerful reasons against number 2 (such as performance, or the effort to affect the change), it sure seems like the nicer way to go. So back to my original question: what other concerns are there, and has anybody done any benchmarks? Con for #2 is a showstopper. Forget about it. -- --Guido van Rossum (python.org/~guido)

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