google app engine - Idiomatic List Wrapper -

in google app engine, make lists of referenced properties this:

class referenced(basemodel):     name = db.stringproperty()  class thing(basemodel):     foo_keys = db.listproperty(db.key)      def __getattr__(self, attrname):         if attrname == 'foos':             return referenced.get(self.foo_keys)         else:             return basemodel.__getattr__(self, attrname) 

this way, can have thing , thing.foos , legitimate out of it. problem comes when says thing.foos.append(x). not save added property because underlying list of keys remains unchanged. wrote solution make easy append keys list:

class keybackedlist(list):     def __init__(self, key_class, key_list):         list.__init__(self, key_class.get(key_list))         self.key_class = key_class         self.key_list = key_list      def append(self, value):         self.key_list.append(value.key())         list.append(self, value)  class thing(basemodel):     foo_keys = db.listproperty(db.key)      def __getattr__(self, attrname):         if attrname == 'foos':             return keybackedlist(thing, self.foo_keys)         else:             return basemodel.__getattr__(self, attrname) 

this great proof-of-concept, in works expected when calling append. however, never give other people, since might mutate list in other ways (thing[1:9] = whatevs or thing.sort()). sure, go define __setslice__ , whatnot, seems leave me open obnoxious bugs. however, best solution can come with.

is there better way trying (something in python library perhaps)? or going wrong way , trying make things smooth?

if want modify things this, shouldn't changing __getattr__ on model; instead, should write custom property class.

as you've observed, though, creating workable 'referencelistproperty' difficult , involved, , there many subtle edge cases. recommend sticking list of keys, , fetching referenced entities in code when needed.