How to recursively traverse a tree and create a list of visited nodes in python

Question

I have defined a class Tree which consists of a list of TreeNodes as below:

class Tree(object):
    def __init__(self, name, nodes):
        self.name = name
        self.nodes = nodes

class TreeNode(object):
    def __init__(self, name, parent):
        self.name = name
        self.parent = parent

As you can see, for each TreeNode I only define a single parent node. However, I want to write a Tree method that gives me a list of all parent nodes of a target node called targetNodeName (the output list should also include targetNodeName itself). For this, I have written a recursive function that iterates until a node with no parent is found (ie the root node) while building a list called results.

def allParents(self, targetNodeName, results):
    currentNode = next((node for node in self.nodes if node.name == targetNodeName))
    results.append(currentNode.name)
    if (currentNode.parent == None):
        print results
        return results
    else:
        return results.append(self.allParents(currentNode.parent, results))

However my recursive function does not perform as intended. I give an example where I first define a three-level, 7-node tree and then call the allParents method to get all parent nodes of node 'N7' ie ['N7', 'N3', 'N1'].

# create nodes
myTreeNodes = []
myTreeNodes.append(TreeNode(name = 'N1', parent = None))
myTreeNodes.append(TreeNode(name = 'N2', parent = 'N1'))
myTreeNodes.append(TreeNode(name = 'N3', parent = 'N1'))
myTreeNodes.append(TreeNode(name = 'N4', parent = 'N2'))
myTreeNodes.append(TreeNode(name = 'N5', parent = 'N2'))
myTreeNodes.append(TreeNode(name = 'N6', parent = 'N3'))
myTreeNodes.append(TreeNode(name = 'N7', parent = 'N3'))

myTree = Tree(name = 'ST1', nodes = myTreeNodes)

a = myTree.allParents(targetNodeName = 'N7', results = [])
print a

> ['N7', 'N3', 'N1']
> None

Although it prints out the correct list of parent nodes -note the 'debug' print command in the function- (ie ['N7', 'N3', 'N1']), the function returns None, meaning that i am dropped off the function with nothing to return. How can I fix this?

Answer 1

Stepping through the method using a debugger is very helpful in working out the path that the code takes.

Using this I can see that the method initially follows the else branch, it is only the child call to self.allParents that causes the print statement. The problem lies with results.append which always returns None and not the list.

a = []
b = a.append(3)
assert a == [3]
assert b is None

The simplest solution would be to split the line into the original results.append , followed by return results .

Answer 2

Use is for checking if the value equals None or not. The allParents method can be simplified as following:

def allParents(self, targetNodeName):
    currentNode = next(node for node in self.nodes
                       if node.name == targetNodeName)
    if currentNode.parent is None:
        return [currentNode.name]
    else:
        return [currentNode.name] + self.allParents(currentNode.parent)