[Python-ideas] An even simpler customization of class creation

[Andrew Barnert]

On Mar 6, 2015, at 5:21, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 4 March 2015 at 03:28, Ethan Furman <ethan at stoneleaf.us> wrote:
>> On 03/02/2015 02:57 AM, Martin Teichmann wrote:
>>>    class Soliton(type):
>>>       def __new__(cls, name, bases, ns):
>>>           self = super().__new__(name, bases, ns)
>>>           return self()
>>> 
>>>    class A(metaclass=Soliton):
>>>        def f(self):
>>>            print(__class__)
>> 
>> This should be `self.__class__`, yes?
> 
> No, it's printing out the type referenced from the definition time
> cell*, rather than the type of the object actually passed in to the
> method.
> 
> Cheers,
> Nick.
> 
> P.S. *For anyone that isn't already aware, the general gist of how that works:
> 
> * The compiler actually special cases "super()" and "__class__" in method bodies
> * If it sees either of them, it implicitly adds a "__class__" cell
> reference to the function
> * In the "super()" case, the call is implicitly rewritten as
> "super(<first param>, __class__)"
> * __class__ is later filled in with a reference to the class being
> defined, once that actually exists

I think it's clearer to explain it as:

Every method has a nonlocal variable named __class__ that refers to the class it's defined in. (As an optimization, CPython doesn't create a closure cell for __class__ if the method body doesn't need it.)

The only problem with this definition is that someone implementing Python and writing type.__new__ has to know at exactly what point the __class__ cell becomes available. But I don't think an implementation that made it available too early would be considered broken, would it?

> In my view, that's never going to win any awards for "elegance in
> language design", but it makes super() so much easier to use I'm happy
> to tell my sense of aesthetics to be quiet :)
> 
> -- 
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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