class NodeItem:
    def __init__(self, val=None, prev=None, next=None):
        self.val = val
        self.prev = prev
        self.next = next

    def add_node(self, value):
        # step through linked list until at last element
        scan = self
        while scan.next:
            scan = scan.next

         # add new element at end
        scan.next = NodeItem(value, prev=scan)

    def __str__(self):
        values = []
        scan = self
        while scan:
            values.append(str(scan.val))
            scan = scan.next
        return ', '.join(values)

def main():
    test = NodeItem(-1)
    x = 42
    while x != 0:
        x = int(input("enter number, or 0 to exit: "))
        test.add_node(x)
    print(f'test: {test}')

main()