Python Sets

Overview

Sets store unique, hashable items with no guaranteed order. They excel at deduplication, membership tests, and set operations like union and intersection.

Syntax / Usage

# Create
tags = {"python", "web", "api"}
from_list = set([1, 2, 2, 3])  # {1, 2, 3}
empty = set()  # not {} which is a dict

# Add and remove
tags.add("devops")
tags.discard("web")  # no error if missing
tags.remove("api")   # KeyError if missing

# Membership (fast)
"python" in tags  # True

# Set operations
a = {1, 2, 3}
b = {3, 4, 5}
a | b   # union: {1, 2, 3, 4, 5}
a & b   # intersection: {3}
a - b   # difference: {1, 2}

Examples

Find users in both groups:

group_a = {"alice", "bob", "carol"}
group_b = {"bob", "dave", "eve"}
both = group_a & group_b  # {"bob"}

Remove duplicates while preserving rough order (use dict keys in 3.7+):

items = ["a", "b", "a", "c", "b"]
unique = list(dict.fromkeys(items))  # ["a", "b", "c"]

Common Mistakes

• Using {} for an empty set—it creates an empty dict
• Adding unhashable items like lists to a set
• Expecting sets to maintain insertion order in older Python versions
• Using sets when you need indexed access—use a list instead

See Also

python-dictionaries python-tuples python-list-comprehension