Code Snippet Explainer

Summary: this function returns the unique items from a list while preserving order. Walkthrough: it creates an empty set seen and an empty result list; it loops over each item; if the item is not in seen, it adds it to seen and appends it to result; finally it returns result. Complexity: O(n) time and O(n) space, since set lookups are constant time. Gotcha: items must be hashable — passing a list of lists would raise a TypeError.