[Urwid] How do you use SimpleListWalker?

Ian Ward ian at excess.org
Sat Jun 20 10:28:06 EDT 2009

What you describe should work, but I'm not a fan of mixing such
different interfaces in the same class.

I would use a slightly modified SimpleListWalker class for storing your
list of conversations, and give each of your conversation objects a
button object (or whatever widget you like) stored as ".widget"

class ThreadedMessageWalker(SimpleListWalker):

    def get_focus(self):
        if len(self)==0: return None, None
        return self[self.focus].widget, self.focus

    def get_next(self, start_from):
        pos = start_from+1
        if len(self) <= pos: return None, None
        return self[pos].widget, self.pos

    def get_prev(self, start_from):
        pos = start_from-1
        if pos < 0: return None, None
        return self[pos].widget, self.pos

This way your list updates automatically, and the ListBox only sees the


Dominic LoBue wrote:
> I see, thanks for the heads up.
> I did some tests while writing my message threading module and as far
> as I can tell trying to do threading piecemeal is many times slower. I
> suppose instead of threading all the messages from scratch every
> single time the program is loaded I can cache the results of the
> threader with pickle and just load that instead, but that still
> doesn't help the fact that if you have a lot of email, the result is
> going to be a lot of threads.
> Right now the threader stores results in an internal list that it can
> modify on the fly so new messages found can be inserted into the
> thread with as little fuss as possible. The results are a simple list
> with each thread being a custom dict class I made called that looks
> like this: {'references':[], 'subjects':[], 'labels': [],
> 'messages':[]}. Inside of references, subjects, and labels is just a
> list of strings. Inside messages are the results my Whoosh index,
> which is a dict of all the headers from an email.
> So I don't have to apply widgets to each message in the threaded list,
> or make a copy of my threaded list which I'd then have to somehow keep
> in sync with the master list, I thought I'd just add the button widget
> methods directly to my conversation dict class. That way instead of
> doing [urwid.Text(x) for x in threadedlist] I just do
> threadmodule.convClass = convClasswithButtonMethods.
> I suppose I can also modify the internal list structure that my
> threading class holds to instead be a SimpleListWalker so I don't have
> to load the results into that when I'm done.
> Other than having to now muck around with metaclasses, do you forsee
> any performance problems with how I'm trying to do this? Or is there a
> better way that I just don't know about?
> Dominic
> On Fri, Jun 19, 2009 at 11:39 AM, Ian Ward<ian at excess.org> wrote:
>> Dominic LoBue wrote:
>>> What makes SimpleListWalker inappropriate for larger lists? For a quick test
>>> I used Text rather than Button, but I'm able to scroll through all 2600
>>> conversation threads rather quickly. No CPU spikes or anything.
>> There are all the usual reasons for not wanting big lists, which are
>> implemented as arrays in python:
>> - Inserting items is O(n)
>> - Deleting items is O(n)
>> - Must fit in memory*
>> Also for Urwid you will need to create all your widgets for every item
>> in advance, which can introduce delays.
>> There should be no performance problem when scrolling the ListBox, it is
>> oblivious to where the widgets are coming from.  With custom ListWalker
>> classes you do need to think about this though, because you can
>> introduce delays with long calculations in get_next or get_prev.  The
>> browse.py program has this problem when it reads directories for the
>> first time.  Ideally it should create a temporary "Loading..." widget
>> and replace it when done loading and sorting.
>> Ian
>> * python gets a bad reputation because of programmers that don't think
>> about this when working with huge data structures, for example old
>> versions of "yum"
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