queues.py

class OurQueue:

    def __init__(self):
        self.in_stack = [] # tail
        self.out_stack = [] # head

    def __len__(self):
        return len(self.in_stack) + len(self.out_stack)

    def push(self, obj):
        self.in_stack.append(obj)
        
    def pop(self):
        if not self.out_stack: # head is empty

            # Note that the in_stack is assigned to the out_stack
            # in reverse order. This is because the in_stack stores
            # elements from oldest to newest whereas the out_stack
            # needs to pop elements from newest to oldest
        
            self.out_stack = self.in_stack[::-1]
            self.in_stack = []
        
        return self.out_stack.pop()