[Tutor] assign all parameters of __init__ to class variables?

[Alex Hall]

I'm sorry, I misspoke (well, mistyped anyway). I have a couple
class-level variables, but most of them are set in the __init__ so
that every instance gets a fresh copy of them. Thatnks for the
responses.

On 11/2/11, Steven D'Aprano <steve at pearwood.info> wrote:
> On Thu, 3 Nov 2011 12:57:25 am Alex Hall wrote:
>> Thanks to both of you, that will work. I can see the argument for
>> assigning everything manually, but I have each parameter on its own
>> line with a comment to explain what it does, and I know I want all
>> parameters to be class-level for every instantiation, so saying
>> "self.[varName]=[varName]" twenty times seems like a waste of time
>> and space.
>
> Wait, you you want the parameters to be *class level*, that is, shared
> by all instances? But you want them to be set each time you
> instantiate an instance? That's fairly unusual. Normally people want
> each instance to get its own instance-level attributes.
>
> In Python, one way of doing this would be:
>
> class K(object):
>     a = 1  # class-level shared attribute defined once only
>     def __init__(self, b):
>         self.__class__.b = b  # Set the shared class-level attribute
>
>
> And in action:
>
>>>> ham = K(23)
>>>> ham.a, ham.b
> (1, 23)
>>>> eggs = K(42)
>>>> ham.a, ham.b
> (1, 42)
>
>
> Notice that you must assign the attribute on the class object itself,
> not the instance. So if you have a bunch of parameters, rather than
> updating the instance dictionary, you have to update the class
> dictionary. (This makes error checking and parameter validation even
> more important, because you can screw your class up by overwriting
> methods if you pass the wrong parameter name.)
>
> But assuming you like living dangerously, or you trust the caller not
> to be stupid:
>
> class K(object):
>     a = 1
>     def __init__(self, **kwargs):
>         self.__class__.__dict__.update(kwargs)
>
> But having done that, it still won't really work the way you expect
> (or at least the way I *think* you will expect). The problem is that
> the instance still has its own dict, so when the caller assigns to an
> attribute:
>
> instance = K(b=2, c=3)
> instance.d = 4
>
> b and c become class-level, as requested, but d is instance-level and
> any other instances will not share that value.
>
> The best way to ensure all instances share the same attributes is with
> the Borg design pattern:
>
> http://code.activestate.com/recipes/66531/
>
> (Also known as Monostate, or the poorly-named "stateless proxy".)
>
> The above recipe is for classic classes, which no longer exist in
> Python 3; for a new-style class version, see the comment by Alex
> Naanou on that page. The other alleged new-style Borgs actually
> implement Singletons.
>
> But if the difference between a Singleton and a Borg matters to you,
> you're probably up to no good. <wink>
>
>
>
> --
> Steven D'Aprano
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> To unsubscribe or change subscription options:
> http://mail.python.org/mailman/listinfo/tutor
>


-- 
Have a great day,
Alex (msg sent from GMail website)
mehgcap at gmail.com; http://www.facebook.com/mehgcap