defaultdict auto-create on access

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Accepted Answer

After the three append calls, `d` has keys 'a' and 'b', so its length is 2. But accessing `d['c']` doesn't raise a KeyError - defaultdict creates the key with the default factory (an empty list) and returns it. After that access, 'c' is now in `d`, making the length 3. The tricky part is that just reading `d['c']` mutates the dict by adding the key.