Sets — Unique Collections

Sets — Unique Collections

A set is an unordered collection of unique values. Use sets when you only care about membership ("Is this item here?") and need to eliminate duplicates.

fruit_colors = {"red", "green", "yellow", "red"}
# Set automatically removes duplicate "red"
# Result: {"red", "green", "yellow"}

Creating Sets

s = {1, 2, 3}             # set literal
s = set([1, 2, 2, 3])     # from list (removes duplicates)
s = set("hello")          # from string → {'h', 'e', 'l', 'o'}

Set Operations (for Data Filtering)

users_bought_a = {1, 3, 5, 7}
users_bought_b = {2, 3, 6, 7, 8}

# Intersection: bought BOTH A and B
users_bought_a & users_bought_b  # {3, 7}

# Union: bought EITHER A or B
users_bought_a | users_bought_b  # {1, 2, 3, 5, 6, 7, 8}

# Difference: bought A but NOT B
users_bought_a - users_bought_b  # {1, 5}

Set Methods

s = {1, 2, 3}
s.add(4)              # {1, 2, 3, 4}
s.remove(2)           # {1, 3, 4} — raises KeyError if not found
s.discard(2)          # {1, 3, 4} — no error if not found
s.pop()               # remove & return arbitrary element
1 in s                # True (O(1) lookup!)

When to use sets: Filtering duplicate IDs, finding common elements between datasets, membership testing.