attribute assignment effects all class instances

[anon]

I'm aware that you can assign a value to an attribute in all class
instances by assigning to <Class>.<attribute>, however, my case is
slightly different and bizarre.

In module node:

top, left, mid, right = range(4)

class Node:

def __init__(self, ntype = None, function = None, symbol = None,
meta = None, adjacent = [None, None, None, None]):
self.type = ntype
self.function = function
self.symbol = symbol
self.meta = meta
self.adjacent = adjacent
Here's the wierd thing, in another module I have:

node.adjacent[left] = y

where node is an instance of the Node class. This statement assigns
all Node instances the value y to attribute adjacent[left]. This seems
very wrong, unless python resolves names in a case-insensitive way.

When I try the alternate:

node.adjacent = [None, y, None, None]

I get the behavior I expect (ie. only the instsance node gets its
attribute set)

Can anyone shed any light on this?

Thanks.
--Greg

[Jeffrey Froman]

anon wrote:

Here's the wierd thing, in another module I have:

node.adjacent[left] = y

where node is an instance of the Node class.**This*statement*assigns
all Node instances the value y to attribute adjacent[left].**This*seems
very wrong, unless python resolves names in a case-insensitive way.
When you assign the default value for adjacent in the constructor definition
as you do, that list is created only one time, and the same list is passed
as the default value for each instance of Node.

Thus, when you alter the list, the change shows up in all instances, because
they are all using the same list.
When I try the alternate:

node.adjacent = [None, y, None, None]

I get the behavior I expect (ie. only the instance node gets its
attribute set)

In the second case you have not altered the existing list, but rather
reassigned your attribute to a new list. Hence, only the node you are
working on changes. At that point all of the nodes *except* the one you
just changed are using the same list for the "adjacent" attribute.
Hope that helps,
Jeffrey

[anon]

In article <n4********************************@4ax.com>, Dennis Lee
Bieber <wl*****@ix.netcom.com> wrote:

On Thu, 09 Sep 2004 16:59:24 -0700, anon <an**@anon.net> declaimed the
following in comp.lang.python:

In module node:

class Node:

Here's the wierd thing, in another module I have:

node.adjacent[left] = y

where node is an instance of the Node class. This statement assigns

I'd recommend some name changes somewhere... Too many "nodes"
here...

Your "another module" had to have done an "import node" (since
that is the name you claim at the top). You then had to perform
something like: "node = node.Node()" to create the instance, but that
would also supersede the module name node...

Dennis--
I actually imported the module as "from node import *", so I haven't
run into a name conflict (apparently). Though I see your point for
naming things differently.

Peter--
Sorry for my miswording of assignment to shared class data.

Everyone--
Thank you very much for your assistance. I was unaware that the
default arguments were created only once and then reused for each
instance. The situation makes complete sense now.

--Greg