When would you use a deep copy instead of a shallow copy?

Using a deep copy is particularly important when you want to copy an object that contains nested objects. A deep copy creates a new instance of the object and recursively copies all objects found within that structure. This means that changes made to any of the nested objects in the deep copy do not affect the original object and vice versa.

In contrast, a shallow copy creates a new instance of the object but does not create copies of the nested objects; it merely copies references to them. As a result, if any changes are made to the nested objects through the shallow copy, those changes will reflect in the original object, as both copies are sharing the same nested elements.

When dealing with complex data structures, such as lists of lists or dictionaries containing lists, it's essential to use a deep copy to ensure that the integrity and independence of the objects are maintained, especially when modifying any of these nested objects. This makes it clear why the choice that involves copying an object with nested objects directly aligns with the purpose of deep copying.