问是什么原因导致我的linkedlist队列python文件中的name error错误无法工作？

EN

# queue is empty at beginning

queue = {'front': None, 'back': None}

# we will use a node to keep track of the elements
# in the queue which is represented by a linked list

class node:
    def __init__(self, data, next):
        self.data = data
        self.next = next

# add elements to queue in 0(1) time
    def enqueue(queue, element):
        N = node(element, None)
        if queue['back'] == None:
            queue['front'] = N
            queue['back'] = N
        else:
            queue['back'].next = N
            queue['back'] = queue['back'].next

# remove first element from queue in 0(1) time
def dequeue(queue):
    if queue['front'] != None:

        first = queue['front']
        queue['front'] = queue['front'].next
        return first.data
    else:
        if queue['back'] != None:
            queue['back'] = None
        return 'Cannot dequeue because queue isempty'

enqueue(queue, 'a')
enqueue(queue, 'b')
enqueue(queue, 'c')

print (dequeue(queue))