[Python-Dev] A new dictionary implementation

[Guido van Rossum]

On Wed, Feb 1, 2012 at 9:13 AM, Hans Mulder <hansmu at xs4all.nl> wrote:
> On 30/01/12 00:30:14, Steven D'Aprano wrote:
>>
>> Mark Shannon wrote:
>>>
>>> Antoine Pitrou wrote:
>
> [......]
>
>>> Antoine is right. It is a reorganisation of the dict, plus a couple of
>>> changes to typeobject.c and object.c to ensure that instance
>>> dictionaries do indeed share keys arrays.
>>
>>
>>
>> I don't quite follow how that could work.
>>
>> If I have this:
>>
>> class C:
>> pass
>>
>> a = C()
>> b = C()
>>
>> a.spam = 1
>> b.ham = 2
>>
>>
>> how can a.__dict__ and b.__dict__ share key arrays? I've tried reading
>> the source, but I'm afraid I don't understand it well enough to make
>> sense of it.
>
>
> They can't.
>
> But then, your class is atypical.  Usually, classes initialize all the
> attributes of their instances in the __init__ method, perhaps like so:
>
> class D:
>    def __init__(self, ham=None, spam=None):
>        self.ham = ham
>        self.spam = spam
>
> As long as you follow the common practice of not adding any attributes
> after the object has been initialized, your instances can share their
> keys array.  Mark's patch will do that.
>
> You'll still be allowed to have different attributes per instance, but
> if you do that, then the patch doesn't buy you much.

Hey, I like this! It's a subtle encouragement for developers to
initialize all their instance variables in their __init__ or __new__
method, with a (modest) performance improvement for a carrot. (Though
I have to admit I have no idea how you do it. Wouldn't the set of dict
keys be different while __init__ is in the middle of setting the
instance variables?)

Another question: a common pattern is to use (immutable) class
variables as default values for instance variables, and only set the
instance variables once they need to be different. Does such a class
benefit from your improvement?

> -- HansM

-- 
--Guido van Rossum (python.org/~guido)