Dictionaries offer one often-used mutation method, setdefault. The setdefault method behaves exactly like the get method except that the returned value is associated with the given key if that key is not in the dictionary. For instance, {'one': 1, 'two': 2}.setdefault('two') behaves just like {'one': 1, 'two': 2}.get('two'), returning 2. But {'one': 1, 'two': 2}.setdefault('three') not only returns None, it also updates the dictionary to be {'one': 1, 'two': 2, 'three': None}. Likewise, {'one': 1, 'two': 2}.setdefault('three', 100) changes the dictionary to {'one': 1, 'two': 2, 'three': 100} before returning 100.

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