[Python-Dev] reducing self.x=x; self.y=y; self.z=z boilerplate code (original) (raw)

Josiah Carlson jcarlson at uci.edu
Sat Jul 2 09:12:48 CEST 2005

"Ralf W. Grosse-Kunstleve" <rwgk at cci.lbl.gov> wrote:

Josiah Carlson wrote: > Now, don't get me wrong, definining slots can be a pain in the > tookus, but with a proper metaclass, that metaclass can define the > slots attribute based on the argument list for init(). > > There you go.

Where? The meta-class idea sounds interesting. Could you work it out?

I had assumed that you were a 'go-getter', and that you would want to figure it out yourself. Apparently I was wrong.

Being that I don't use metaclasses (I don't need the functionality), I had to spend 10 minutes learning about them, and 5 minutes implementing the following.

class AutoSlots(type): def init(cls, name, bases, dct): slots = dict.fromkeys(dct.get('slots', [])) if 'init' in dct: init = dct['init'] ifvn = init.func_code.co_varnames for i in xrange(init.func_code.co_argcount): x = ifvn[i] if x[:1] == '_'and x[1:] not in slots: slots[x[1:]] = None if slots: dct['slots'] = slots.keys() super(AutoSlots, cls).init(name, bases, dct)

def InitSlots(ob, args): for k, v in args.items(): if k[:1] == '_': setattr(ob,k[1:],v)

class Foo(object): metaclass = AutoSlots def init(self, a, b, _x, _y=None): InitSlots(self, locals())

foo = Foo(1,2,3) vars(foo) {'y': None, 'x': 3}

> A syntax change is wholly unnecessary.

I wonder why everybody gets so agitated about a syntax enhancement proposal. I am not proposing a syntax change!

Yes you are. Any thing that changes syntax, necessarily is a syntax change. People get "agitated" because with every syntax addition, that is just another syntax that a newbie may need to learn in order to understand some block of code. Further, for those of us who try to teach about it, it is just one more little nit that students ask about.

Considering that EVERYTHING you want is possible with 17 lines of support code (which can be tossed into site and assigned to builtins), and 2 lines of boilerplate (which can be made into one metaclass line if you are willing to do a bit more work), a syntax change is foolishness.

I know enhancing the syntax is work, but shouldn't a syntax leading to less code clutter be the higher value?

Why bother if the non-syntax-change goes 99% of the way? I've further pushed myself to -10 for any syntax change offering during my implementation of AutoSlots.

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