如何在Python中使所有引用变量都为None？

Question

I got a typical ListNode class something like below,

class ListNode:
    def __init__(self,val, next = None):
        self.val = val
        self.next = next

I created two instances of this class, and assigned one of the object to a variable called next in first instance as below,

node = ListNode(1)
next_node = ListNode(2)
node.next = next_node

However, if I assign nnode = None , the node.next still points to the instance next_node and is not None .

print( next_node == node.next) # Prints True
next_node = None
print( node.next.val) # Prints 1

How can I make all the reference variables (eg node.next in above case) make None without explicitly assigning them to None?

Answer 1

I really like your question. Probably this is not exactly what you are looking for, but I have found weakref library. With it you could try to modify your code:

class ListNode:
    def __init__(self, val, next_=None):
        self.val = val
        self.next_ = next_

    def get_next(self):
        try:
            self.next_.check()
        except AttributeError: # if self.next_ exists then it does not have 'check' method.
            return self.next_
        except ReferenceError: # if it does not exists it will raise this exception
            return None

Then instead of assigning node.next_ = next_node , you can create weakref:

node = ListNode(1)
next_node = ListNode(2)
node.next_ = weakref.proxy(next_node)

print( next_node == node.next_) # prints True
next_node = None
print(node.get_next()) # prints None

Hope this will help solve your problem!

Edit: I have also changed the name next to next_ because next is a builtin function