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Dictionaries are not for fixed fields.

if you have a dict, it maps something to something else. You should be able to add and remove values.

Objects, on the other hand, are supposed to have specific fields of specific types, because their methods have strong expectations of what those fields and types are.

attrs lets you be specific about those expectations; a dictionary does not. It gives you a named entity (the class) in your code, which lets you explain in other places whether you take a parameter of that class or return a value of that class.