I want to delete a class instance or make it None in python. But I'm not able to do it. In general my code does something as follows:-

class Foo:
  pass

f = Foo()
g = Foo()
f.attr = g
h = f.attr
del h
print f.attr  # This is not None but it should be right?

More specifically this problem came while implementing binary search trees in python. My delete function does something like this:-

class Tree:
  def delete(self, key):
    min_right = self.right.getmin()
    self.key = min_right.key
    del min_right  # Does not work

Am I doing something wrong or is it expected behavior? If it is expected, what is the best way by OOP standards to achieve what I am trying to do?

You have a situation that looks like this:

                  g

                  |

     <__main__.Foo object at 0x...>

          /               \

       f.attr               h

i.e. three references to the same underlying object. All that del h does is remove the name h, like this:

                  g

                  |

     <__main__.Foo object at 0x...>

          /               

       f.attr

The underlying object doesn't change, and the two other names still provide a reference to it, all you've done is remove one reference to the Foo instance. h is now a NameError, but g and f.attr will work just fine.

 # This is not None but it should be right?

I don't know why you would conclude that - if you did succeed in breaking the reference from f.attr to the Foo instance, you would get an AttributeError, not None.

Recommended reading on Python names: http://nedbatchelder.com/text/names.html