How to implement a generator in a Linked List class?

Question

I am trying to implement a simple Linked List in Python, and sort of trying to use all the Python concepts that I learned in it. I am stuck in implementing a Generator for the class.

Code:

def __iter__(self):
    return self
    

def next(self):
    tempNode = self.head
    while tempNode:
        yield tempNode.data
        tempNode = tempNode.nextNode
    else:
        raise StopIteration

Usage:

list_gen = iter(list1)
print (next(list_gen))
print (next(list_gen))

Output:

generator object next at 0x7ff885b63960

generator object next at 0x7ff885b63960

Neither is it printing the data value of the node, nor is it maintaining the current state of the method [as evident from the addresses returned].

Where am I making the mistake? Thanks in advance.

EDIT I modified the code to:

def next(self):
    tempNode = self.head
    while tempNode:
        tempNode2 = tempNode
        tempNode = tempNode.nextNode
        return tempNode2.data
    else:
        raise StopIteration

Now, the node value is printed, but as mentioned earlier, the state is not preserved and the value of the first node is printed every time.

Have you seen this answer? https://stackoverflow.com/a/42983747/5606265 — Tekay37, Feb 12 '18 at 14:17

user2390182 · Accepted Answer · 2018-02-12T14:35:14.380

You need an instance variable to maintain the iterator's state between calls to next(). Also, __next__ is not a generator function, it merely returns the next element, so do not use yield there:

def __init__(self, ...):
    # ...
    self.iter_node = None  # maintain state of iterator

def __iter__(self):
    return self

def next(self):
    if self.iter_node is None  # reset iterator to allow repeated iteration
        self.iter_node = self.head or 'XXX'  # dummy for to signal end
    if self.iter_node != 'XXX':
        rval = self.iter_node.data
        self.iter_node = self.iter_node.nextNode or 'XXX'
        return rval
    else:
        self.iter_node = None
        raise StopIteration

Thanks, your suggestion helped! Solved the issue. – Nilashish C Feb 12 '18 at 14:56 — Nilashish C, Feb 12 '18 at 14:56

How to implement a generator in a Linked List class?

1 Answers1