Figuring out what class instance contains you

[Alex VanderWoude]

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.

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__()).

I have tried using inspect.getmembers() and walking up the
sys._getframe() stack, but to no avail. Am I missing something here?
Or is it simply not possible for the NewDict instance to figure out for
itself what its container is?

 

[Gabriel Genellina]

En Fri, 04 Apr 2008 00:41:19 -0300, Alex VanderWoude

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.