Multiple levels of collection defaultdict in Python

In the example above, the defaultdict is created with the default factory set to int, so any missing keys will be automatically initialized to 0.
In this example, the nested_dict lambda function creates a defaultdict that uses itself as the default factory. This allows us to create a nested structure of defaultdicts, making it easy to access and modify values at different levels.
Let's create a practical example using nested defaultdicts to count the frequency of words in a list of sentences.
In this example, the word_frequency defaultdict is used to count the occurrences of each word. The nested structure allows us to easily navigate through the levels of the dictionary.
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