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Date: Thu, 1 Mar 2012 22:25:11 -0800
Subject: Re: Is this the proper way to use a class method?
From: Chris Rebert <clp2@rebertia.com>
To: John Salerno <johnjsal@gmail.com>
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On Thu, Mar 1, 2012 at 10:00 PM, John Salerno <johnjsal@gmail.com> wrote:
> This is purely for fun and learning, so I know there are probably better =
ways of creating a chess program. Right now I'm just curious about my speci=
fic question, but I'd love to hear any other advice as well.
>
> Basically, I'm wondering if I'm using the class method properly with the =
move method.

Unfortunately, no.

> The reason I did it this way is because every chess piece will obviously =
have its own move method, yet they will all need to mark the piece as moved=
, so I figure that's best written once in the superclass.

Right, but why not just a regular instance method? The method in
question doesn't operate upon *the class itself* at all (which is the
raison d'etre of classmethods); it instead operates upon an instance.
A staticmethod would be slightly less weird, but really, it should
just be an instance method.

> This works, but doing it this way seems weird, since the point of a class=
 method is that it can be called by the class itself, even before instances=
 have been created. Yet the way I've implemented it, it is necessarily tied=
 to being called on an instance. Is this wrong? Is there a better way to do=
 what I'm doing with move?

Just make it a normal instance method (i.e. remove the `cls` argument).

> Also, do I need the @classmethod decorator? The book I'm reading says to =
use it (and @staticmethod), but the following code works without it.

That's just a coincidence. Your supercall is ought to be: super().move()
In contrast, super().move(self) calls the superclass instance method
`move` with 2 arguments, both `self`, which just happens to work given
your move() method, inside which `cls` isn't actually a class like it
ought to be.

<snip>
> class ChessPiece:
>
> =C2=A0 =C2=A0def __init__(self, position, label, has_moved):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0try:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0self.position =3D (position[0], =
int(position[1]))
> =C2=A0 =C2=A0 =C2=A0 =C2=A0except TypeError:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0self.position =3D position
> =C2=A0 =C2=A0 =C2=A0 =C2=A0self.label =3D label
> =C2=A0 =C2=A0 =C2=A0 =C2=A0self.has_moved =3D has_moved
>
> =C2=A0 =C2=A0def move(cls, self):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0self.has_moved =3D True
>
>
> class Pawn(ChessPiece):
>
> =C2=A0 =C2=A0def __init__(self, position=3DNone, label=3D1, has_moved=3DF=
alse):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0super().__init__(position, label, has_moved)
>
> =C2=A0 =C2=A0def move(self):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0super().move(self)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0self.position =3D (chr(ord(self.position[0]) +=
 1), self.position[1] + 1)

Cheers,
Chris