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Style question: metaclass self vs cls?

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  Style question: metaclass self vs cls? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 15:29 +0000
    Re: Style question: metaclass self vs cls? Terry Reedy <tjreedy@udel.edu> - 2012-07-16 18:21 -0400
    Re: Style question: metaclass self vs cls? alex23 <wuwei23@gmail.com> - 2012-07-16 23:10 -0700
      Re: Style question: metaclass self vs cls? Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-17 10:35 -0600
    Re: Style question: metaclass self vs cls? Michele Simionato <michele.simionato@gmail.com> - 2012-07-17 05:23 -0700
      Re: Style question: metaclass self vs cls? Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-17 10:34 -0600
      Re: Style question: metaclass self vs cls? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-18 02:50 +0000

#25422 — Style question: metaclass self vs cls?

Here's a style question for you: in a metaclass, what should I call the 
instance parameter of methods, "cls" or "self"?

class ExampleMeta(type):
    def method(self, *args): ...

I'm not quite sure if that feels right. On the one hand, self is the 
ExampleMeta instance alright... but on the other, self is actually a 
class, so I feel I want to call it "cls" rather than "self", which makes 
it more obvious that you're looking at a metaclass.

On the third-hand, it may be confusing that the argument is called "cls" 
but not decorated with classdecorator.

I'm very slightly leaning towards writing metaclasses like this:

class ExampleMeta(type):
    def __new__(meta, *args): ...
    def method(cls, *args): ...

class Example(metaclass=ExampleMeta):
    def another_method(self): ...


What do others do?



-- 
Steven

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#25444

On 7/16/2012 11:29 AM, Steven D'Aprano wrote:
> Here's a style question for you: in a metaclass, what should I call the
> instance parameter of methods, "cls" or "self"?
>
> class ExampleMeta(type):
>      def method(self, *args): ...
>
> I'm not quite sure if that feels right. On the one hand, self is the
> ExampleMeta instance alright... but on the other, self is actually a
> class, so I feel I want to call it "cls" rather than "self", which makes
> it more obvious that you're looking at a metaclass.

I have never seriously written a metaclass, but as a reader I would 
prefer 'cls'.

> On the third-hand, it may be confusing that the argument is called "cls"
> but not decorated with classdecorator.

To me, that reinforces 'looking as a metaclass'.

An @classmethod in a class is a class method specific to the particular 
class. A method in a metaclass is a method common to all classes of the 
metaclass. They could be written differently, yet calling the first 
param 'cls' either way seems reasonable.

> I'm very slightly leaning towards writing metaclasses like this:
>
> class ExampleMeta(type):
>      def __new__(meta, *args): ...
>      def method(cls, *args): ...
>
> class Example(metaclass=ExampleMeta):
>      def another_method(self): ...

> What do others do?

Not too many people write real metaclasses. Python lets you chose. Have 
you looked at the C code of type? (Not that you are bound by it.)

-- 
Terry Jan Reedy

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#25464

On Jul 17, 1:29 am, Steven D'Aprano <steve
+comp.lang.pyt...@pearwood.info> wrote:
> Here's a style question for you: in a metaclass, what should I call the
> instance parameter of methods, "cls" or "self"?

Maybe portmanteu it as "clasself"? :)

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#25511

On Tue, Jul 17, 2012 at 12:10 AM, alex23 <wuwei23@gmail.com> wrote:
> On Jul 17, 1:29 am, Steven D'Aprano <steve
> +comp.lang.pyt...@pearwood.info> wrote:
>> Here's a style question for you: in a metaclass, what should I call the
>> instance parameter of methods, "cls" or "self"?
>
> Maybe portmanteu it as "clasself"? :)

What is this, 1st edition D&D?  Elf is a race, not a class.

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#25492

The standard is to use `cls`. In the __new__ method you can use `mcl` or `meta`.

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#25510

On Tue, Jul 17, 2012 at 6:23 AM, Michele Simionato
<michele.simionato@gmail.com> wrote:
> The standard is to use `cls`. In the __new__ method you can use `mcl` or `meta`.

I've also seen `mcs` a fair amount.

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#25553

On Tue, 17 Jul 2012 05:23:22 -0700, Michele Simionato wrote:

> The standard is to use `cls`. In the __new__ method you can use `mcl` or
> `meta`.

Thanks to everyone who answered.

I think I will stick with "meta" and "cls".


-- 
Steven

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