dataclass field with default_factory

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Accepted Answer

Using field(default_factory=list) means a brand-new list is created for each Basket instance. This is different from writing items: list = [], which would share one list object across all instances - the classic mutable-default bug. When a.items.append('apple') modifies a's list, b's list is untouched because they are separate objects. default_factory accepts any zero-argument callable, so you can use dict, set, or your own factory function for other mutable defaults.