Which is faster and why? Set or List?

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Membership testing in a set is vastly faster, especially for large sets. That is because the set uses a hash function to map to a bucket. Since Python implementations automatically resize that hash table, the speed can be constant (O(1)) no matter the size of the set (assuming the hash function is sufficiently good).

In contrast, to evaluate whether an object is a member of a list, Python has to compare every single member for equality, i.e. the test is O(n).

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