Adding Nodes to a graph displays object instead of string (adjacency list)

Question

I'm learning how to create a graph using an adjacency list on Python. My current problem is when trying to add a node to the list, it displays the Node object at 0x0000.... instead of a string. When I try to print out the list, I get TypeError: list indices must be integers or slices, not Node". I can't seem to figure out a way to fix this. Any help would be appreciated!

class Node:
    def __init__(self, name):
        self.name = name
        self.visited = False
        self.adjacency = []

    def addNeighbor(self, v):
        if v not in self.adjacency:
            self.adjacency.append(v)


class DGraph:
    def __init__(self, size=20):
        self.size = size
        self.numNodes = 0
        self.nodeList = [0] * size

    def addNode(self, name):
        """adds new node to graph"""
        if self.numNodes >= self.size:
            raise OverflowError("Graph Size Exceeded")
        newNode = Node(name)
        newNode.name = name
        newNode.addNeighbor(name)
        self.nodeList[self.numNodes] = newNode
        self.numNodes += 1

    def listNodes(self):
        theList = "Nodes: "
        for i in self.nodeList:
            theList += self.nodeList[i]
            theList += ""
        return theList


tree = DGraph()
tree.addNode("A")
tree.addNode('C')
tree.addNode('T')

What the list looks like in the debugger

Yes, I linked a screenshot of the list in the debugger below the code. When I try to list the nodes, I get "TypeError: list indices must be integers or slices, not Node" — Sage, Nov 20 '22 at 04:25

score 1 · Accepted Answer · answered Nov 20 '22 at 04:41

You can specify string representation of your object by implementing __repr__ See details in the docs and this question Here is a working example (nodeList is fixed too)

class Node:
    def __init__(self, name):
        self.name = name
        self.visited = False
        self.adjacency = []

    def addNeighbor(self, v):
        if v not in self.adjacency:
            self.adjacency.append(v)

    def __repr__(self):
        return self.name

    def __str__(self):
        return self.name



class DGraph:
    def __init__(self, size=20):
        self.size = size
        self.numNodes = 0
        self.nodeList = [0] * size

    def addNode(self, name):
        """adds new node to graph"""
        if self.numNodes >= self.size:
            raise OverflowError("Graph Size Exceeded")
        newNode = Node(name)
        newNode.name = name
        newNode.addNeighbor(name)
        self.nodeList[self.numNodes] = newNode
        self.numNodes += 1

    def listNodes(self):
        theList = "Nodes: "
        for i in self.nodeList:
            theList += str(i)
            theList += " "
        return theList


tree = DGraph()
tree.addNode("A")
tree.addNode('C')
tree.addNode('T')
print(tree.listNodes())

Now debugger will show node names (using__repr__)

Such implementation of __repr__ is probably not a good idea unless you enforce uniqueness of node names.

As a side note, it's nice to follow naming convention in Python

I understand now, thank you! I also have another question. I created a function that finds and returns the index of A. For example: `return self.nodeList.index(name)`. When I try to pass in `tree.findNode("A")`, I get "ValueError: "A" is not in the list. I noticed that inside of Node A, there's a string named "A". Is there a way to access the string "A" that is inside of Node A? I hope my explanation is clear enough to understand. — Sage, Nov 21 '22 at 18:50
Because there are no String "A" in your `nodeList` You have an object of type `Node` that has `name="A"`. If you think that the name is a unique for all nodes, then you can store your nodes in a dictionary instead of a list. Dictionary will have a key "A" and Node as a value. — BlacKow, Nov 22 '22 at 00:00
Glad I could help. Looks like its's a season of adjacency questions :) — BlacKow, Nov 22 '22 at 00:23

Adding Nodes to a graph displays object instead of string (adjacency list)

1 Answers1