Recursion Depth Exceeded - Python Doubly-Linked List

Question

I'm in the process of creating a FIFO implemented as a doubly-linked list, but I'm at a loss as to why I'm receiving a recursion error. I've posted my code and the error I received below. Any help would be much appreciated!

"""DLList.py"""

class DLList(object):
    """A doubly-linked list

    attributes: nl = node list"""
    def __init__(self, nl=[]):
        self.nl = nl

    def __str__(self):
        return str(self.nl)

    def __len__(self):
        return len(self.nl)

    def append(self, node):
        if not isinstance(node, DLNode):
            raise TypeError
        try:
            node.pn = self.nl[-1]
            node.pn.nn = node
            self.nl.append(node)
        except:
            self.nl.append(node)

    def pop(self):
        rn = self.nl.pop(0)
        try:
            self.nl[0].pn = None
        except:
            pass
        return rn


class DLNode(object):
    """A node in a doubly-linked list.

    attributes: self.pn = previous node, self.data = data, self.nn = next node"""
    def __init__(self, data):
        self.pn = None
        self.data = data
        self.nn = None

    def __str__(self):
        return '[%s, %s, %s]' % (self.pn, self.data, self.nn)


a = DLNode(17)
b = DLNode(15)
c = DLNode(12)
d = DLNode(46)
e = DLNode(46)

ages = DLList()
ages.append(a)
ages.append(b)
ages.append(c)
ages.append(d)
ages.append(e)

print ages.pop()

I received this error:

File "C:\python\swampy-2.0\DLList.py", line 43, in __str__
    return '[%s, %s, %s]' % (self.pn, self.data, self.nn)
RuntimeError: maximum recursion depth exceeded

The thing is, I never intended to use recursion, and I can't figure out why I've entered into a recursive loop. ages.pop() is meant to return an instance of DLNode.

Answer 1

When you print the node, you are trying to print self.pn and self.nn. Each of those is a reference to another node, which when printed will try to print its next node and previous node, and so on, ad infinitum. You are asking to print an infinite hall of mirrors of nodes.

You have nothing keeping track of what has already been printed, so the recursion will continue forever.

Answer 2

Maybe I recommend using collections.deque instead? You can read its documentation in the library. Unless you are writing your implementation as an academic exercise, deque is probably better.