changing how instances are "created"

[John Roth]

"newseater" <kbilsted at hotmail.com> wrote in message 
news:1118620840.352981.60420 at g14g2000cwa.googlegroups.com...
>> > class Creator
>> > def createInstance(cls, *args, **kwargs):
>> > anewinstance = cls.__new__(cls, *args, **kwargs)
>> > anewinstance.__init__(*args, **kwargs)
>> >
>> > return anewinstance
>> > createInstance = staticmethod(createInstance)
>> >
>> >
>> > can anyone help??
>>
>> __new__ is the proper way to do this, but making it work
>> is a bit tricky. If you could post the code you tried to
>> get to work for __new__(), we could critique it.
>>
>> The current documentation is in 3.3.1 of the Python
>> Reference Manual (Python 2.4 version). In earlier
>> versions, there were a couple of other documents
>> that did a better (IMO) job of explaining things.
>>
>> The trick is that __new__ must return an instance.
>> It can be a newly created instance (and the doc shows
>> how to do this) or an existing instance of any new style
>> class.
>
> from the documentation it was very hard to guess what the "..." was
> supposed to be. also i wasn't able to find out how to control if
> __init__() would be called on an instance (if i reused that instance..
> and that instance could be of a completely different type).

I believe that __init__() is not called if the instance is not of
the same type as the class with the __new__() method.

There was a thread on that not too long ago, and I believe
that was the resolution: the docs were wrong, __init__ is
only called if the returned instance is for the class with the
__new__ method. The 2.4 docs seem to say it properly.

> eg. i didn't get much out of
> http://docs.python.org/ref/customization.html
>
> from http://gnosis.cx/publish/programming/metaclass_1.html

This has nothing to do with metaclasses. Wandering into
that will fry your brain.

...

>
> but that way of creating objects is just plain silly!

Yep. If you don't need a metaclass, don't use one. Metaclasses
are a special purpose construct. If you need it, you'll know that
you need it. Othewise you don't, and you certainly don't here.

> I tried looking at the __new__ buildin but this seems to be addressing
> old-style objects (which i'm not interested in using).

__new__ is only invoked for new style classes; it is not invoked
for old style classes.f

The following may be helpful: it's a relatively extended article
on new style classes that was left out of the 2.4 references
because the content had supposedly been absorbed into the
mainline documentation.

http://www.python.org/2.2.3/descrintro.html


>> By the way - when you post code, please use spaces
>> for indentation. There are a number of popular mail
>> clients that don't play fair with tabs, and people using
>> these clients will frequently ignore code that isn't
>> properly indented.
>
> sorry. maybe people should complain to those lousy newsreader authors
> instead of everyone having to comply to the lowest standards? I guess
> the readers are actually un*x readers although your comment typically
> is found in the w*ndows world where everyone is forced into using word
> :-)

This dead horse has been beaten into a pulp many times. People
with newsreaders and mail clients that don't play fair with tabs are
not going to change; snide remarks will only reflect on the person
making the remark.

The major problem is IE, although I've seen other newsreaders
and mail clients do the same thing. Also, I don't even have Word
on my Windows system; I use Open Office. Being "forced" to
use Word is a choice.

John Roth
>
>
>> John Roth
>