[Python-Dev] subclassing builtin data structures

[Mark Roberts]

> On Feb 12, 2015, at 18:40, Chris Angelico <rosuav at gmail.com> wrote:
> 
> On Fri, Feb 13, 2015 at 12:46 PM, MRAB <python at mrabarnett.plus.com> wrote:
>>>>> class BaseInt:
>> ...     def __init__(self, value):
>> ...         self._value = value
>> ...     def __add__(self, other):
>> ...         return type(self)(self._value + other)
> 
>> On Fri, Feb 13, 2015 at 11:55 AM, Guido van Rossum <guido at python.org> wrote:
>> ... there is no reason (in general) why
>> the signature of a subclass constructor should match the base class
>> constructor, and it often doesn't.
> 
> You're requiring that any subclass of BaseInt be instantiable with one
> argument, namely its value. That's requiring that the signature of the
> subclass constructor match the base class constructor.
> 
> ChrisA
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No, it seems like he's asking that the type return a new object of the same type instead of one of the superclass. In effect, making the Date class call type(self)(*args) instead of datetime.date(*args). He seems completely willing to accept the consequences of changing the constructor (namely that he will have to override all the methods that call the constructor).

It seems like good object oriented design to me.

-Mark