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Welcome back everybody.

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You've already seen that you can use square brackets multiple

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times to extract an item from deep inside a nested data structure.

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Often, you'll want to traverse all the stuff that's in

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a nested data structure or at least some subset of it.

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Perhaps, you'll print all the items out or accumulate them into a flat, un-nested list.

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In this lesson, you'll do that through nested iteration,

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a for-loop indented inside a for-loop

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indented inside a for-loop as many levels as you need.

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We'll also look at the confusions that can occur from a list or

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dictionary being nested inside other lists or dictionaries.

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It's analogous to the problems of aliasing that we've seen before,

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where multiple variables point to the same object.

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If you mutate a list say by appending an extra item,

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you've effectively changed all the lists and dictionaries that contain that list.

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We'll introduce the notion of deep as opposed to

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shallow copies of nested data structures.

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Finally, we'll introduce the understand, extract,

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repeat method of incrementally figuring

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out how to extract data from a nested data structure.

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At the end of this lesson,

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you should be able to write nested for-loops,

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to traverse a nested data structure,

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recognize when an inner list or dictionary is shared among multiple outer objects,

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and if necessary make deep copies instead of shallow copies to avoid that,

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and you should be able to incrementally develop code to extract the data you want from

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a complex nested data structure using the understand, extract, repeat process.

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Bye for now. We'll see you at the end.