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Lesson 6 — Sets: dedupe while keeping order
Sets for fast "have I seen this?"
Checking x in some_list looks at every item — slow for big lists. x in some_set is instant. A common pattern: remove duplicates while keeping the original order, using a set to remember what you have already seen.
seen = set()
seen.add("a")
"a" in seen # True
(set(list) also removes duplicates, but it loses the order.)
Why it matters: cleaning duplicate emails or IDs from a sign-up list is a daily task, and the seen-set pattern does it in one fast pass.
Want it another way?
Task
Inside the if, add the email to both seen and result. Expected: 3 unique emails in original order.
Run your code — when the output matches the task, you will see a green check. Then mark the lesson complete.
Test your knowledge
Test your knowledgeA quick 3-question quiz on this lesson — instant feedback.