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。

This post explains how to add to a dictionary in Python, create new dictionaries, and more. Powerful data manipulation is now within your grasp!

The core of the Python data model architecture is the special methods (also known as 'magic methods'). These methods, which ...