Local variable referenced before Python

Question

I have the following code:

def popLast(aLinkedList):
    ptr = aLinkedList
    while ptr != None:
        if ptr['next']['next'] == None and ptr['next'] != None:
            del_node = ptr['data']
            ptr['next'] = ptr['next']['next']
        ptr = ptr['next']

    return (del_node,ptr)

When I put

myLinkedList = createList(['a', 'b', 0, [1, 2]])
for i in range(getLength(myLinkedList)):
    (a, myLinkedList) = popLast(myLinkedList)
print(a, end = ", new list: ")
printList(myLinkedList)

It keeps giving me

UnboundLocalError: local variable 'del_node' referenced before assignment

When I just use popLast(myLinkedList), there is no problem. Everything works fine. I don't know whether it is because my returning tuple is the problem.

The reference to createlist is not explained in your code. Please give us a Minimal, Complete, and Verifiable example. — Rory Daulton
– Rory Daulton, Commented Nov 12, 2016 at 0:56
Note: PEP8 suggests that you should use while ptr is not None instead of while ptr != None. — Phrogz
– Phrogz, Commented Nov 12, 2016 at 0:57

acushner · Accepted Answer · 2016-11-12 01:21:05Z

1

If an empty list is passed in, then del_node is never initialized. This is what is causing the error. To change this, first initialize del_node to None:

def popLast(aLinkedList):
    ptr = aLinkedList
    del_node = None
    while ptr is not None:
    ...

edited Nov 12, 2016 at 1:21

acushner

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answered Nov 12, 2016 at 0:59

Michael Schultz

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