Flatten a list

Python Advanced astronaut ages teen+

The problem

Write a function flatten(items). The list items can hold numbers and also other lists, and those lists can hold lists too, as deep as you like. Give back one flat list with all the numbers in the same order. Example: flatten([1, [2, [3]], 4]) gives [1, 2, 3, 4].

You will practice: lists inside lists, isinstance, recursion (a function calling itself)

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Flatten a list

The problem

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