Creating an object that can track when its attributes are modified

Chris Angelico rosuav at gmail.com
Wed Mar 6 18:03:59 CET 2013


On Thu, Mar 7, 2013 at 3:56 AM, Ben Sizer <kylotan at gmail.com> wrote:
> On Wednesday, 6 March 2013 16:22:56 UTC, Chris Angelico  wrote:
>>
>> Effectively, you would need to have a
>> subclass of list/dict/tuple/whatever that can respond to the change.
>
> This is certainly something I'd be interested in having, but I guess that would be fragile since the user would have the burden of having to remember to use those types.

Since you're already overriding setattr, you could simply force all
non-string sequences to your special subclass of list. That reduces
that burden, though it'd break if there are any other references to
the object.

>> What's the goal of this class? Can you achieve the same thing by
>> using, perhaps, a before-and-after snapshot of a JSON-encoded form of
>> the object?
>>
>
> I need to be able to perform complex operations on the object that may modify several properties, and then gather the properties at the end as an efficient way to see what has changed and to store those changes. Any comparison of before-and-after snapshots could work in theory, but in practice it could be expensive to produce the snapshots on larger objects and probably expensive to calculate the differences that way too. Performance is important so I would probably just go for an explicit function call to mark an attribute as having been modified rather than trying to do a diff like that. (It wouldn't work for rollbacks, but I can accept that.)

Hmm. Interesting. The perfect solution probably is too messy, yeah.
But if you have your subclassing done, you could possibly
snapshot-on-write, which would allow the rollback. Not sure if it'd
help though.

ChrisA



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