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Sets

A set is an unordered, unindexed collection of items with no duplicate elements. Sets are written with curly braces {}.

1. Set Basics and Deduplication

names = ["Alice", "Bob", "Alice", "Charlie", "Bob"]

# Removing duplicates by casting to a set
unique_names = set(names)
print(unique_names)  # Output: {'Alice', 'Bob', 'Charlie'} (unordered)

# Converting back to a list
unique_names_list = list(unique_names)

2. Adding and Removing Elements

  • add(): Adds an item.
  • remove(): Removes an item (raises a KeyError if item doesn't exist).
  • discard(): Removes an item safely (does NOT raise an error if item is missing).
s = {1, 2, 3}
s.add(4)
s.discard(99)  # Does nothing safely

3. Fast Membership Testing

Checking if an item exists using in is extremely fast for sets compared to lists, especially for large datasets.

allowed_users = {"admin", "staff", "editor"}
print("admin" in allowed_users)  # Output: True (runs in O(1) constant time)

4. Mathematical Set Operations

Sets support standard mathematical operations like Union, Intersection, Difference, and Symmetric Difference:

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

# Union (all items from both sets)
print(a | b)  # Output: {1, 2, 3, 4, 5, 6}
# Or: a.union(b)

# Intersection (common items)
print(a & b)  # Output: {3, 4}
# Or: a.intersection(b)

# Difference (in a but not in b)
print(a - b)  # Output: {1, 2}
# Or: a.difference(b)

# Symmetric Difference (items in either a or b, but NOT both)
print(a ^ b)  # Output: {1, 2, 5, 6}
# Or: a.symmetric_difference(b)

Further Reading