queue3.py

class Queue3:
    def __init__(self):
        self.items = []
        self.end = 0

    def enqueue(self, item):
        self.items.append(item)

    def dequeue(self):
        item = self.items[self.end]
        self.items[self.end] = None
        self.end += 1
        return item

    def size(self):
        return len(self.items) - self.end

    def is_empty(self):
        if self.items[-1] is None:
            return True
        else:
            return False

    def cleanup(self):
        self.items = self.items[self.end:]
        self.end = 0