Sets store unique, hashable elements with no guaranteed order, and support fast membership tests and mathematical set operations.
python
seen = {"0xabc", "0xdef"}
seen.add("0x123")
"0xabc" in seen # O(1) average
a = {1, 2, 3}
b = {2, 3, 4}
a & b # intersection {2, 3}
a | b # union {1, 2, 3, 4}
a - b # difference {1}Sets are commonly used to deduplicate a list: `unique = list(set(addresses))` (note this loses original order).