Hybrid list property: append not calling setter

[Avi Blackmore]

I have a hybrid property with a custom setter defined for a one-to-many relationship on one of my classes, and for assignments using the = operator, it works fine.  I can assign a new list of objects to that property, and it calls the setter to handle the details.

But when I use the append() method on the list, the setter is not called.

How do I change this behavior?  I would like to be able to append, so I don't have to do silly things like obj.property = obj.property + [new].  That's not very Pythonic.

Thanks,

Avi Blackmore

[Mike Bayer]

we have to consider ordinary Python mechanics here. A hybrid
property is a Python descriptor, meaning it plugs in at the point at
which you set, access, and delete an attribute. A collection is
just any other value. But when we look into .append(), descriptors
have no access to that, if we look at:

myobject.attribute.append(foo)

that is really:

my_collection = myobject.attribute # <-- descriptor.__get__ is called
my_collection.append(foo) # <-- has nothing to do with the descriptor

so intercepting the append() has to do with what you're doing. if
you're returning the collection directly from a relationship attribute
then just use the append event:

http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.AttributeEvents.append

that event will be called for all new collection items even if a new
list is assigned. you also might want to use the bulk_replace event:
http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.AttributeEvents.bulk_replace
depending on what you're trying to do.



>
> Thanks,
>
> Avi Blackmore
>
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[Avi Blackmore]

That is very helpful, thank you!  I will hook into the append event, I think.

Thanks very much.

Avi Blackmore