implement a class Python-CodePudding

I'm studying Python and have an exercise about classes Python. According to the exercise, I have to implement a class of Node, that initialize 2 attribute name and neighbors which satisfy the condition:

def test_node(self):
    node = Node(name="a", neighbors=[Node(name="b")])
    assert node
    assert getattr(node, "name") == "a"
    assert len(getattr(node, "neighbors")) == 1

But I have trouble to understand the attribute neighborsin the line node = Node(name="a", neighbors=[Node(name="b")]). Could someone explain to me?

CodePudding user response：

Regarding your question "What does node = Node(name="a", neighbors=[Node(name="b")]) do?":

Node (name="a") or Node (name="b") create an instance of class Node with the respective names.
neighbors=[Node(name="b")] creates a list, containing an instance of Node with the name b. As this is passed to the creation of an instance of Node with the name a, in total you will get two instances (one with name a, which has a neighbor in a list named b as well as an instance of Node named b, without a neighbor).

I hope that clarifies your actual question.

CodePudding user response：

Here is a little demonstration that might be of help:

def test_node():
    node = Node(name="a", neighbors=[Node(name="b")])
    assert node
    assert getattr(node, "name") == "a"
    assert len(getattr(node, "neighbors")) == 1


class Node():
    def __init__(self, name, neighbors=[]):
        self.name = name
        self.neighbors = neighbors

    def add_neighbor(self, neighbor):
        self.neighbors.append(neighbor)

test_node() # No error means passes all checks

# Demonstration
a = Node(name='a') # create node a having name 'a' without any neighbors
b = Node(name='b') # same as above
c = Node(name='c', neighbors=[a,b]) # This node c has a and b as neighbors

print(a.__dict__) # see all node's attributes and their current values

a.add_neighbor(b) 
a.add_neighbor(Node(name='d')) # create a node d and add as a neighbor to a
print(a.__dict__) # now a has b and d as neighbors

Run this code and tinker with it to get a better understanding. I'm leaving the type checking as an exercise, as it will be needed to ensure that neighbors are all also of the Node class.

CodePudding user response：

In your implementation, neighbours is a list composed of str variables which correspond to the names of the neighbour Nodes. However, question asks you to create a list of Nodes that are the neighbours of our Node. That is to say, you have to put the Node objects themselves to your list, not their names.

Apparently, Node(name="b") is creating a new Node object with the given name. What you have to do is to create a constructor that takes a name parameter and also a list of neighbours (optionally). If list is not provided, then we should understand that that specific Node doesn't have any neighbours.

One way to do it:

from typing import List, Optional

class Node:
    def __init__(self, name, neighbors: "Optional[List[Node]]" = None):
        self.neighbors = neighbors if neighbors is not None else []
        self.name = name

    def add_neighbor(self, node: "Node"):
        self.neigbors.append(node)
        return self.neighbors
    def __str__(self):
        return "Node object named as '{}'".format(self.name)
def test_node():
    node = Node(name="a", neighbors=[Node(name="b")])
    print(node)
    print(getattr(node, "name") == "a")
    print(len(getattr(node, "neighbors")) == 1)
        
test_node()

Output:

Node object named as 'a'
True
True