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• A dedicated reverse_list method is not really necessary.

• An __iter__ method is more useful than a dedicated print_list method (thanks ChatterOne).

• Circular lists are also reversed neatly this way (note that __iter__ cycles endlessly for circular lists).

• A dedicated reverse_list method is not really necessary.

• An __iter__ method is more useful than a dedicated print_list method (thanks ChatterOne).

• Circular lists are also reversed neatly this way (note that __iter__ cycles endlessly for circular lists).

class DoubleLinkedListNode:
 def __init__(self, value, plus_node, dir=None):
 self.value = value
 self.minus_node = None
 if plus_node:
 plus_node.minus_node = self
 if dir == None:
 self.dir = plus_node.dir
 if dir != None: 
 self.dir = dir
 self.plus_node = plus_node
 def __iter__(self):
 self.cur = self
 return self
 def __next__(self):
 if self.cur:
 current = self.cur
 if self.cur.dir:
 if self.cur.dir[0] > 0:
 self.cur = self.cur.plus_node
 elif self.cur.dir[0] < 0:
 self.cur = self.cur.minus_node
 return current
 else:
 raise StopIteration
if __name__ == "__main__":
 tail = DoubleLinkedListNode(5, None, [1])
 head = DoubleLinkedListNode(1, DoubleLinkedListNode(2, DoubleLinkedListNode(3, DoubleLinkedListNode(4, tail))))
 for i in head: print(i.value)
 head.dir[0] = -1 # reverses direction for all nodes in the list
 for i in tail: print(i.value)

class DoubleLinkedListNode:
 def __init__(self, value, plus_node, dir=None):
 self.value = value
 self.minus_node = None
 if plus_node:
 plus_node.minus_node = self
 if dir == None:
 self.dir = plus_node.dir
 if dir != None: 
 self.dir = dir
 self.plus_node = plus_node
 def __iter__(self):
 self.cur = self
 return self
 def __next__(self):
 if self.cur:
 current = self.cur
 if self.cur.dir:
 if self.cur.dir[0] > 0:
 self.cur = self.cur.plus_node
 elif self.cur.dir[0] < 0:
 self.cur = self.cur.minus_node
 return current
 else:
 raise StopIteration
if __name__ == "__main__":
 tail = DoubleLinkedListNode(5, None, [1]) # [1] sets direction for all nodes in list.
 head = DoubleLinkedListNode(1, DoubleLinkedListNode(2, DoubleLinkedListNode(3, DoubleLinkedListNode(4, tail))))
 for i in head: print(i.value) # Prints node values forwards from head.
 head.dir[0] = -1 # Reverses direction for all nodes in the list.
 for i in tail: print(i.value) # Prints node values in reverse from tail.

I have improved the code to handle these issues below. Preventing an endless loop requires keeping track of the identity of each node. I have done this by incrementing a global variable node_count, but there are more Pythonic ways to keep a count of instances of objects.

Below is an example of how \$\mathcal{0}(1)\$ reversal of double-linked lists might be implemented using a direction flag. Points relevant to OP's code are:

• A dedicated ́reverse_list ́ method is not really necessary.

• An ́iter ́ method is more useful than a dedicated ́print_list ́ method (Thanks ).

• Circular lists are also reversed neatly this way (note that ́iter ́ cycles endlessly).

Your code could be easily improved to handle these issues. Preventing an endless loop requires keeping track of the identity of each node. Below, I've done this by incrementing a global variable node_count, but there are more Pythonic ways to keep a count of instances of objects.

Below is an example of how \$\mathcal{O}(1)\$ reversal of double-linked lists might be implemented using a direction flag. Points relevant to OP's code are:

• A dedicated reverse_list method is not really necessary.

• An __iter__ method is more useful than a dedicated print_list method (thanks ChatterOne).

• Circular lists are also reversed neatly this way (note that __iter__ cycles endlessly for circular lists).