[Tutor] Acting on objects in a list - how to make the change permanent?

[Kent Johnson]

Adam,

You are confusing a "list of things that should be deleted" with the list 
from which they should be deleted.

For example suppose I have the list [1, 2, 3, 4, 5]:
 >>> a=[1, 2, 3, 4, 5]
 >>> a
[1, 2, 3, 4, 5]

Now I get a list of things to delete from a:
 >>> d=[1, 3]

Deleting something from d will not be helpful!
 >>> d.remove(1)
 >>> a
[1, 2, 3, 4, 5]
 >>> d
[3]

I have to remove from a:
 >>> d=[1, 3]
 >>> for i in d:
...   a.remove(i)
...
 >>> d
[1, 3]
 >>> a
[2, 4, 5]

OK, now in your real code, you have a nested structure of lists - a 
magazine collection has titles which have issues which have articles. So 
it's not so simple, when you have just the list d, to figure out what list 
to delete from - it could be any of the issues in any of the titles.

The solution is to remember which list the item to be deleted occurs in. 
Say we have two lists:
 >>> a=[1, 2, 3, 4, 5]
 >>> b=[3, 4, 5, 6]

If I tell you to remove item 3, what will you do? But if I tell you to 
remove item 3 from list b, you know what to do. So the list of things to 
delete has to include information about the list to delete from. Say we 
want to remove 3 from b and 4 from a:
 >>> d=[ (3, b), (4, a) ]

Now d is a list of tuples. Each tuple tells me an item to delete from a 
specific list. Here's how to remove them:
 >>> for val, lst in d:
...   lst.remove(val)
...
 >>> a
[1, 2, 3, 5]
 >>> b
[4, 5, 6]

You need to do something like this. Your list of things to remove has to 
include a reference to the list from which to remove it.

Kent

At 03:01 PM 9/27/2004 +0100, Adam Cripps wrote:
>My python project (http://savannah.nongnu.org/projects/newmag) is
>teaching me a fair bit about how to write python code. However, I'm
>still a long way away from being remotely good, as some of my recent
>posts have shown.
>
>I'm working on deleting instance objects and I've created a list of
>objects which is then piped through a menu to allow the user to choose
>which one is deleted.
>
>The basic structure of the code is this:
>
>  * choose_what_to_edit  - creates a list which calls return_object_list
>      * return_object_list - return_object_list  -chooses what level
>the user wants to select an object
>         * choosetitle - selects a title
>             * chooseissue - if necssary selects an issue from the 
> selected title
>                * choosearticle - and if nescessary, selects an article
>from the selected issue.
>
>Basically, from this code, I get a list, which has objects within it.
>If I alter the objects within this list, how do I make them permanent?
>I'm currently trying to delete an item/object of the list the command
>del returned_list[choice] - however, this doesn't seem to be a
>permanent change. For the full code listing, please see the CVS
>project page.
>
>Am I right in thinking that the list of objects will be referenced
>through to the actual instances, and if so, why aren't they being
>deleted? I've read some of the python docs, but can't see the solution
>here and would appreciate a more informed/specialised approach.
>
>TIA.
>Adam
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