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

[Steven D'Aprano]

On Mon, Mar 02, 2015 at 11:57:09AM +0100, Martin Teichmann wrote:

> Imagine a soliton-generating metaclass:
> 
>     class Soliton(type):
>        def __new__(cls, name, bases, ns):
>            self = super().__new__(name, bases, ns)
>            return self()

You have to pass cls as an explicit argument to __new__, otherwise you 
get a TypeError:

TypeError: type.__new__(X): X is not a type object (str)

(that's in Python 3.3). The line should be:

self = super().__new__(cls, name, bases, ns)


 
> And generate such a soliton:
> 
>     class A(metaclass=Soliton):
>         def f(self):
>             print(__class__)
> 
> As of now, writing "A.f()" interestingly prints "<__main__.A object>", so
> __class__ is indeed set to what Soliton.__new__ returns, the object,
> not the class.

Is "soliton" the usual terminology for this? Do you perhaps mean 
singleton? I've googled for "soliton" but nothing relevant is coming up.

 
> This is currently correct behavior, but I think it actually is not what one
> expects, nor what one desires. (Does anyone out there make use of
> such a construct? Please speak up!) 

I do now! Seriously, I think you have just solved a major problem for 
me. I'll need to do some further experimentation, but that is almost 
exactly what I have been looking for. I've been looking for a way to 
have a class statement return a custom instance and I think this might 
just do it.


-- 
Steve