[PyQt] question about dip observe handler

[Phil Thompson]

On Mon, 19 Jul 2010 11:13:14 -0400, Darren Dale <dsdale24 at gmail.com>
wrote:
> On Mon, Jul 19, 2010 at 9:33 AM, Phil Thompson
> <phil at riverbankcomputing.com> wrote:
>> On Sun, 18 Jul 2010 18:11:15 -0400, Darren Dale <dsdale24 at gmail.com>
>> wrote:
>>> I am reading the dip documentation at
>>>
>>
http://www.riverbankcomputing.co.uk/static/Docs/dip/complete_example.html
>>> , and have a question about the following:
>>>
>>> "dip will invoke an observe() handler when the value of an attribute
>>> is set, even if the new value is the same as the old one. We therefore
>>> use the _propagate_changes attribute to prevent possible recursion
>>> problems"
>>>
>>> Why is it necessary to invoke the observe handler when the value of an
>>> attribute does not change? There are quite a few infinite-recursion
>>> workarounds in the example. If the possibility of infinite recursion
>>> is so high, perhaps by default the handler should not be invoked if
>>> the attribute value does not change, unless observe() was called with
>>> a kwarg to invoke the handler regardless?
>>
>> Good question.
>>
>> The short answer is that it was easiest for me, but (obviously) what
>> matters is what's easier for developers using it.
> 
> Perhaps the example would be easier to understand (and to develop from
> scratch) with such a change.
> 
>> The issue is to define what is meant by "changed", particularly when
the
>> attribute is a class instance? Rebinding the attribute is obviously a
>> change, but what about a change to an attribute of the instance?
> 
> The last example is another slightly different use case. The way
> enthought supports it is documented at
>
http://code.enthought.com/projects/traits/docs/html/traits_user_manual/notification.html#example-of-a-dynamic-notification-handler

...and what parts of this notation do you find you actually use in
practice?

Phil