This error means that you attempted to index an object that doesn’t have that functionality. You might have noticed that the method sort() that only modify the list have no return value printed – they return the default NoneNone . ‘NoneType’ object is not subscriptable is the one thrown by python when you use the square bracket notation object[key] where an object doesn’t define the getitem method . This is a design principle for all mutable data structures in Python. You can reproduce TypeError that you get in your code if you try this at the Python command line:

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