Singly Linked List: A Python Implementation | Part 2 | __str__ Method

In the previous post, I started by making a Node class.

I also included an __init__ method, to make it easier to instantiate Nodes, and to ensure that each Node has the same attributes.

I then created and printed a node in the following way:

my_node = SLLNode(7, None)
print("Node with value {} and next node {}.".format(my_node.value, my_node.next_node))

That worked, but wouldn't it be nice if we could do something like print(node)?

It turns out we can. We just need to define a __str__ method.

The definition looks like this:

# indented within class SLLNode
    def __str__(self):
        s = "Node with value {} and next node: {}.".format(self.value, self.next_node)
        return s

The method __str__ works under the hood when we call print(my_node) and creates a string representation of my_node.

Let's test it to make sure it's working.

my_node = SLLNode(8, None)
print(my_node)

And the output is:

Node with value 8 and next node: None.

Now, you may have noticed that there is a potential bug lurking under the surface. How will __str__() function when a Node points to another Node? Let's find out.

second_node = SLLNode(27, None)
    first_node = SLLNode(14, second_node)
    print(first_node)

Here's the output:

Node with value 14 and next node: Node with value 27 and next node: None..

Do you see what happened? It might help if I insert some extra formatting into the output, like so:

{Node with value 14 and next node: {Node with value 27 and next node: None.}.}

Turns out __str__ got called recursively; it used __str__ again and called it on the second_node. This isn't necessarily the behaviour we want, because as the list gets longer the output could be really long and cumbersome to read.

I honestly wasn't expecting this. (I did this previously in java, and it behaves a little differently in this regard).

How will we get past this bug? Stay tuned!

The code for this project is available here on GitHub. I'll be updating it gradually as I write this series. I'll be sure to commit at least as often as I post, so you can back up and see that code as it was when I published a given post.

[EDIT]: The next post is now available here. Also, it just occured to me that these posts are themselves effectively a linked list. Meta!