Python Attribute error

Question

having problems with this error in python:

File "F:\dykrstra", line 36, in route
while node.label != node.prevNode.label:
AttributeError: 'NoneType' object has no attribute 'label'

Inside this while loop:

 while node.label != node.prevNode.label:
    node = node.prevNode
    labels.append(node.label)

I think it relates to this:

   def __init__(self, label):
        self.label = label
        self.neighbours = []
        self.distances = []
        self.prevNode = None
        self.totalDistance = 0

I'm not sure why prevNode doesn't like the nothing being assigned to it, please help.

Solution

Your constructor sets self.prevNode to None, and later you try to access node.prevNode.label, which is like trying to access None.label. None doesn't have any attributes, so trying to access any will give you an AttributeError.