defaultdict的核心原理是它在初始化时接受一个工厂函数（factory function），这个函数用于生成默认值。当访问一个不存在的键时，defaultdict会调用这个工厂函数来生成默认值，并将其插入到字典中。这使得defaultdict在处理图算法、计数器等需要频繁检查键是否存在的场景时，比普通字典更加高效和简洁。

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