What is a Python set and what makes it useful for membership testing?

A set is an unordered collection of unique, hashable objects. Internally it is a hash table, giving O(1) average-case lookup — far faster than scanning a list for large collections. Duplicates are silently dropped on creation.

tags = {'python', 'data', 'python', 'api'}  # {'python', 'data', 'api'}

# Membership
print('python' in tags)  # True — O(1)

# Add / remove
tags.add('ml')
tags.discard('api')   # no error if missing (unlike .remove())

# Set operations
a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
print(a | b)   # union     {1, 2, 3, 4, 5, 6}
print(a & b)   # intersection {3, 4}
print(a - b)   # difference   {1, 2}
print(a ^ b)   # symmetric diff {1, 2, 5, 6}

The practical win is deduplication. Converting a list to a set and back is the fastest way to remove duplicates when order does not matter: unique = list(set(my_list)). For order-preserving deduplication use dict.fromkeys(my_list) (dicts maintain insertion order since 3.7).

frozenset is the immutable variant — hashable and usable as a dictionary key or element of another set.

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