Sets

Syntax

{item1, item2, item3}

Examples

numbers = {1, 2, 2, 3, 3, 3}
print(numbers)  # {1, 2, 3} - duplicates removed automatically

empty = set()  # NOT {} - that creates an empty dict instead
print(type(empty))

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 difference: {1, 2, 5, 6}

tags = {"python", "web"}

tags.add("backend")
print(tags)

tags.discard("web")   # no error if 'web' isn't present
print(tags)

print("python" in tags)  # fast membership check

names = ["Fola", "Zain", "Fola", "Jamal", "Zain"]

unique_names = list(set(names))
print(unique_names)  # order not guaranteed

# Preserve original order while deduplicating
seen = set()
ordered_unique = []
for name in names:
    if name not in seen:
        seen.add(name)
        ordered_unique.append(name)

print(ordered_unique)  # ['Fola', 'Zain', 'Jamal']

Best practices

• Use set() to create an empty set - {} creates an empty dictionary instead, a common source of confusion
• Reach for a set when you need fast membership testing (x in my_set) on a large collection - it is much faster than checking a list
• Use sets to deduplicate data, but remember they do not preserve insertion order in general (use a dict or an ordered loop if order matters)
• Only hashable items (numbers, strings, tuples) can go in a set - lists and dictionaries cannot be set members

At a glance

Purpose

Scripting and general-purpose applications

File extension

.py

Runs in

Python interpreter

Usually used with

Python standard library and packages

Specifications & further reading

• Python language reference

Related Python documentation