Figuring out what class instance contains you

[Gabriel Genellina]

En Fri, 04 Apr 2008 00:41:19 -0300, Alex VanderWoude  
<alex at computronix.com> escribió:

> Consider the following module:
>
> ================================
> class NewDict(dict):
>      parent = None
>
>      def __setitem__(self, key, value):
>          print "My parent is", self.parent
>          super(NewDict, self).__setitem__(key, value)
>
> class Zero(object):
>      children = NewDict()
>
>      def __init__(self):
>          self.children.parent = self
>
> class One(Zero):
>      pass
>
> class Two(One):
>      pass
>
> a = One()
> a.children["spam"] = "baked beans"
> b = Two()
> b.children["eggs"] = "bacon"
> ================================
>
> When the above module is executed, the output looks something like this:
>
> My parent is <__main__.One object at 0x00BA27B0>
> My parent is <__main__.Two object at 0x00B9D170>
>
> ...which is great, just what I wanted.  Each dictionary instance reports
> correctly which object instance is its container.

But only if you create a single instance of each class, because "children"  
is a class attribute (behaves as it were shared among all instances).
Then you need an instance attribute, initialized in __init__. So why not  
set the parent at this time?

py> class NewDict(dict): pass
...
py> class Zero(object):
...      def __init__(self):
...          self.children = NewDict()
...          self.children.parent = self
...
py> class One(Zero):
...      pass
...
py> class Two(One):
...      pass
...
py> a = One()
py> a.children["spam"] = "baked beans"
py> b = Two()
py> b.children["eggs"] = "bacon"
py> c = Two()
py> c.children["foo"] = "bar"
py> print a.children.parent is a
True
py> print b.children.parent is b
True
py> print c.children.parent is c
True

> However, I would like to find some automagic way of having NewDict
> figure out what object instance it is on without resorting to having the
> container class instance poke a reference to itself into the NewDict
> instance (i.e. the line in Zero.__init__()).

Unless there are more contrains that you haven't told us, the above  
example looks as the simplest approach.

-- 
Gabriel Genellina