Notify of change to list

Paul Hankin paul.hankin at
Fri Jun 13 05:19:19 CEST 2008

On Jun 13, 12:00 pm, "Alan J. Salmoni" <salm... at> wrote:
> My question is how can my program be notified of a change to a class
> attribute that is a list?

You can't. But you can replace lists inside your class with a list
that notifies changes.

Something like this:

class NotifyingList(list):
    def __setitem__(self, i, v):
        print 'setting', i, 'to', v
        list.__setitem__(self, i, v)

[[You'll likely need to redefine __setslice__, __delitem__,
__delslice__, append and extend as well]].

>>> x = NotifyingList([1, 2, 3])
>>> x[1] = 4
setting 1 to 4
>>> x
[1, 4, 3]

If users of your class are allowed to replace attributes with their
own lists, you'll have to catch these and convert to NotifyingLists;
and it may be somewhat messy.

I hope this is useful to you.

Paul Hankin

More information about the Python-list mailing list