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Sets

Sets are a type of collection in Python that store unique elements. Think of a set like a bag where you can only have one of each item and no duplicates are allowed. Sets are useful when you need to work with unique items or perform operations like finding common elements between collections.

Creating Sets​

You can create a set in Python using curly braces {} or the set() function. Here's how:

# Creating a set with curly braces
fruits = {"apple", "banana", "orange"}

# Creating a set with the set() function
numbers = set([1, 2, 3, 4, 5])

# Creating an empty set
empty_set = set()  # Note: empty_set = {} creates an empty dictionary, not a set

Adding and Removing Elements​

Since sets are mutable (can be changed), you can add and remove elements:

Adding Elements​

fruits = {"apple", "banana"}

# Add a single element
fruits.add("orange")
print(fruits)  # {'apple', 'banana', 'orange'}

# Add multiple elements
fruits.update(["grape", "kiwi"])
print(fruits)  # {'apple', 'banana', 'orange', 'grape', 'kiwi'}

Removing Elements​

fruits = {"apple", "banana", "orange", "grape"}

# Remove an element (raises error if not found)
fruits.remove("banana")
print(fruits)  # {'apple', 'orange', 'grape'}

# Remove an element safely (no error if not found)
fruits.discard("kiwi")  # No error even though 'kiwi' isn't in the set
print(fruits)  # {'apple', 'orange', 'grape'}

# Remove and return a random element
removed = fruits.pop()
print(f"Removed: {removed}")
print(f"Remaining: {fruits}")

# Remove all elements
fruits.clear()
print(fruits)  # set()

Set Operations​

Sets support operations like union, intersection, and difference, which are similar to mathematical set operations.

Union​

A union is the combination of two sets, including all unique elements from both sets:

set1 = {1, 2, 3}
set2 = {3, 4, 5}

# Using the | operator
union = set1 | set2
print(union)  # {1, 2, 3, 4, 5}

# Using the union() method
union = set1.union(set2)
print(union)  # {1, 2, 3, 4, 5}

Intersection​

A intersection contains elements that are common to both sets:

set1 = {1, 2, 3}
set2 = {3, 4, 5}

# Using the & operator
intersection = set1 & set2
print(intersection)  # {3}

# Using the intersection() method
intersection = set1.intersection(set2)
print(intersection)  # {3}

Difference​

A difference contains elements that are only in one set and not in the other:

set1 = {1, 2, 3}
set2 = {3, 4, 5}

# Using the - operator
difference = set1 - set2
print(difference)  # {1, 2}

# Using the difference() method
difference = set1.difference(set2)
print(difference)  # {1, 2}

Checking Membership​

You can also check if an element is a member of a set using the in keyword:

fruits = {"apple", "banana", "orange"}

# Check if an element is in the set
print("apple" in fruits)  # True
print("grape" in fruits)  # False

Set Methods​

Similarly to other data structures, sets have a variety of built-in methods that can be used to manipulate them. Here are some common methods:

fruits = {"apple", "banana", "orange"}

# Get the number of elements
print(len(fruits))  # 3

# Check if a set is a subset of another
# A subset is a set that contains only elements found in another set
small_set = {"apple", "banana"}
print(small_set.issubset(fruits))  # True

# Check if a set is a superset of another
# A superset is a set that contains all elements of another set (and possibly more)
big_set = {"apple", "banana", "orange", "grape"}
print(big_set.issuperset(fruits))  # True

# Check if two sets have no elements in common
# Disjoint sets are sets that have no elements in common
other_set = {"grape", "kiwi"}
print(fruits.isdisjoint(other_set))  # False (they share no elements)

Frozen Sets​

Python also has an immutable version of sets called frozenset. Once created, you cannot add or remove elements:

# Create a frozen set
immutable = frozenset([1, 2, 3])
print(immutable)  # frozenset({1, 2, 3})

# Try to add an element (will raise an error)
try:
    immutable.add(4)
except AttributeError as e:
    print(f"Error: {e}")

Common Use Cases​

Removing Duplicates​

One of the most common uses of sets is to remove duplicates from a list:

numbers = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
unique_numbers = list(set(numbers))
print(unique_numbers)  # [1, 2, 3, 4]

Finding Common Elements​

Sets make it easy to find common elements between collections:

students_in_math = {"Alice", "Bob", "Charlie"}
students_in_science = {"Bob", "Charlie", "David"}

# Find students taking both classes
both_classes = students_in_math & students_in_science
print(both_classes)  # {'Bob', 'Charlie'}

Summary​

Sets are pretty powerful as they allow you to simplify solutions that require working with unique elements or performing operations like finding common elements between collections. They are also great for ensuring that you don't have duplicates in your data structures.