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Sets

Sets are collections of unique elements. They are the go-to tool in Python when we need to remove duplicates or perform fast membership checks (O(1) time).

Use set() to create an empty set, as {} creates an empty dictionary.

Core Set Operations​

Sets are unordered and elements must be immutable (like strings or integers). They are ideal for math-like operations.

Membership and Deduplication​

A set automatically removes any duplicate values we pass into it.

# Duplicates are automatically removed
ingredients = {"ginger", "cardamom", "ginger", "cloves"}
print(ingredients)
# Fast membership check
print("ginger" in ingredients)
{'ginger', 'cloves', 'cardamom'}
True

Union and Intersection​

We can combine sets or find common elements using simple operators.

A = {"apple", "banana"}
B = {"banana", "cherry"}

# Union (|): All unique items from both
print(A | B)

# Intersection (&): Only items present in both
print(A & B)
{'cherry', 'banana', 'apple'}
{'banana'}

Difference and Symmetric Difference​

These help us find what is unique to one set or unique to both.

A = {"apple", "banana"}
B = {"banana", "cherry"}

# Difference (-): Items in A but NOT in B
print(A - B)

# Symmetric Difference (^): Items in A or B, but NOT both
print(A ^ B)
{'apple'}
{'cherry', 'apple'}

Adding and Removing Items​

  • add(): Inserts a single item.
  • update(): Adds multiple items from another collection.
  • discard(): Removes an item safely (doesn't crash if the item is missing).
# Type hinting: specify the element type inside set[]
# See: [Modern Python Types](../1 - basics/1.9 - modern-python-types.md) for details
spices: set[str] = {"pepper"}
spices.add("salt")
spices.update(["cumin", "turmeric"])

# Use discard over remove to avoid KeyErrors
spices.discard("cinnamon")

print(spices)

Output:

{'turmeric', 'salt', 'pepper', 'cumin'}

Immutable Sets (frozenset)​

Standard sets are mutable. If we need a set that cannot be changed—for example, to use it as a key in a dictionary—use frozenset.

# This set cannot be modified after creation
immutable_spices = frozenset(["cardamom", "cloves"])

print(immutable_spices)
frozenset({'cloves', 'cardamom'})

Worth Mentioning: Set Relationships​

Beyond modification, sets have built-in methods to check how they relate to each other.

  • isdisjoint(): Returns True if two sets have no common elements.
  • issubset(): Returns True if all elements of one set are in another.
  • issuperset(): Returns True if one set contains all elements of another.
A = {1, 2, 3}
B = {4, 5, 6}
C = {1, 2}

print(A.isdisjoint(B)) # True (no overlap)
print(C.issubset(A))   # True (C is inside A)
print(A.issuperset(C)) # True (A contains C)

Output:

True
True
True