Tracking Element Changes

[ipython]

Tracking Element Changes

[SylvainCorlay]

Well, we migrated to the new observe API with custom event types in order to support the type of approach that is proposed in #278.

[rmorshea]

@SylvainCorlay, this idea of a WatchedType wouldn't be a full replacement for your pr, but would instead be an alternative to #278's edict where each of the required methods was rewritten in order to catch changes. I've actually written an example that describes an "eventful dict" using WatchedType.

[rmorshea]

@SylvainCorlay, I'll soon push a commit that allows for this kind of work flow (I'll post on your main thread when I'm do):

EventfulList = watched_type("EventfulList", list, '__setitem__')

def pass_on_args(inst, call):
    return call.args

def print_element_change(inst, answer):
    # answer.before = pass_on_args(call)
    index, old = answer.before
    new = inst[index]
    if new != old:
        print("{%s: %s} -> {%s, %s}" % (index, old, index, inst[index]))

elist = EventfulList([1, 2, 3])
elist.notify_before(__setitem__=pass_on_args)
elist.notify_after(__setitem__=print_element_change)

elist[0] = 1
# No print
elist[0] = -1
# PRINTS: '{0: 1} -> {0: -1}'

This represents a generic way to establish callbacks for any method on any type of instance.

I think it is a very powerful, and useful callback tool that avoids the need for hard coded classes like edict or elist.

While this does not replace an EventfulDict trait type, it would be the internal foundation of one.

[rmorshea]

After #317 is merged, this could be a working example:

class HasEvents(HasTraits):

    elist = Eventful(List(), '__setitem__')

    @event(on='elist.__setitem__')
    def elist_set_callbacks(self):
        def before(inst, call):
            m = "set from %s to %s at list index %s"
            print(m % (inst[call.args[0]], call.args[1], call.args[0]))
        return (before, None)

he = HasEvents()
he.elist = [1]
he.elist[0] = 2

Prints: set from 1 to 2 at list index 0