Late initialization using __getattribute__

bukzor workitharder at gmail.com
Thu Sep 4 00:26:10 CEST 2008


On Sep 3, 1:02 pm, Bruno Desthuilliers
<bdesth.quelquech... at free.quelquepart.fr> wrote:
> bukzor a écrit :
> (snip)
>
> > Thanks for the reply. Just to see it not work, I tried to remove
> > __getattribute__ from LateInitMixIn, but couldn't get it to work.
>
> ??? Sorry, I don't get what you mean...

Since you said __getattribute__ is an attribute of the class, I tried
to run (something to the effect of) delattr(self.__class__,
"__getattribute__"), but it threw an AttributeError.


>
> > My Base class is a C class (_mysql.connection from MySQLdb) that
> > sometimes segfaults if you try to use it before it's fully
> > initialized,
>
>
> ... I have used MySQLdb for years on more than a dozen linux
> distribs, and never had such a problem. Is this a known bug ? Or is
> there something wrong with your setup ?

I'm trying to optimize my number of connections by not fully
initializing (read: not connecting) my connection until it's used in
some way. Of course the maintainer didn't envision this (mis)use, so
the object sometimes throws bad errors until it's fully initialized.

Of course the *correct* way to do this is to be more careful about
when I create connections, but I think I should be able to get this to
work, and it (would be) much easier to do it The Right Way once it
works.

> > so unfortunately I think I need to use __getattribute__
> > to do this. I'm doing all this just to make the connection not
> > actually connect until used.
>
> I may be dumb, but I don't get how this is supposed to solve your
> problem. But anyway : there's a known design pattern for what you're
> trying to do, that doesn't require mixins nor messing with
> __getattribute__ (which, I repeat, is more often than not something you
> *don't* want to do). The name of the design pattern is "proxy". I
> strongly suggest that you 1/ try to cure the real problem instead of
> hacking around and 2/ read about the proxy design pattern.
>
> My 2 cents...

I like the idea of mix-ins, but can't figure out how to make a proxy
work that way. For a long time I had a proxy class that added five or
six features on top of the MySQLdb package, but it wasn't configurable
enough, and I'm working on splitting each feature into its own MixIn
class.




As an aside, this is working for me pretty well. The "reconnect"
method (inheritied from a "Reconnectable" mixin) uses several of the
object's attributes, so I need to set _inited beforehand so that I
don't get into an infinite __getattribute__ loop. What I'd *really*
like to do is remove __getattribute__ from the object at that point.
    def __getattribute__(self, attr):
        "connect if it would otherwise cause an error."
        getattr = lambda attr:object.__getattribute__(self, attr)

        if not getattr("_inited"):
            print "connecting."
            setattr(self, "_inited", True)
            getattr("reconnect")()
        return getattr(attr)


Thanks for your help,
--Buck



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