Python set() Function

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The set() function in Python is a built-in function that creates a set object, which is an unordered collection of distinct hashable objects. Sets are akin to formal mathematical sets, making them ideal for membership testing, removing duplicates from a sequence, and performing mathematical operations like unions, intersections, and set differences.


The set() function has a simple syntax:



set() takes a single optional parameter:

iterable – The iterable argument is optional. If provided, the set() function creates a new set containing the elements from the given iterable. If the iterable is not provided, the set() function creates an empty set.

Return Value:

  • an empty set if no parameters are passed
  • a set constructed from the given iterable parameter

Characteristics of Sets

  • Unordered: Sets do not record element position or order of insertion.
  • Mutable: Sets can be changed in place, meaning elements can be added or removed.
  • Unique Elements: Sets store only one instance of an element, thus duplicates are not allowed.
  • Hashable Elements: Elements of a set must be hashable; in other words, they must be immutable, like strings, numbers, and tuples.

Creating Sets with set()

let’s delve into the details of how the set() function is used to create sets from other collections like lists, tuples, and dictionaries. This process is often referred to as “casting” to a set.

Create a Set From Lists

A list in Python is an ordered collection that can contain duplicate elements and is defined by square brackets []. When you cast a list to a set using the set() function, Python creates a new set with all the unique elements from the list. Any duplicate values in the list are removed in the new set.

# Casting a list with duplicate elements to a set
numbers_list = [1, 2, 2, 3, 4, 4, 5]
numbers_set = set(numbers_list)
print(numbers_set)  # Outputs: {1, 2, 3, 4, 5}

In this example, the list numbers_list has duplicate instances of 2 and 4. When we convert this list to a set named numbers_set, these duplicates are eliminated.

Create a Set From Tuples

Tuples are similar to lists in that they are ordered collections, but they are immutable (cannot be modified after their creation). They are defined by parentheses (). When a tuple is converted to a set, the order is disregarded, and, just like with lists, duplicate elements are removed.

# Casting a tuple with duplicate elements to a set
letters_tuple = ('a', 'b', 'a', 'c', 'd', 'b')
letters_set = set(letters_tuple)
print(letters_set)  # Outputs: {'a', 'b', 'c', 'd'}

Here, letters_tuple contains duplicates of a and b. Upon conversion to a set, letters_set, we are left with each element occurring exactly once.

Create a Set From Dictionaries

When you cast a dictionary to a set, you only get the keys of the dictionary. Since dictionary keys are unique (like set elements), this is a natural conversion. Values associated with the keys are not included in the resulting set.

# Casting a dictionary to a set
info_dict = {'name': 'Alice', 'age': 25, 'gender': 'Female'}
info_set = set(info_dict)
print(info_set)  # Outputs: {'name', 'age', 'gender'}

The info_dict dictionary has keys ‘name’, ‘age’, and ‘gender’. When cast to a set info_set, we obtain a set of these keys.

Create a Set From Strings

A string can also be converted to a set, which will consist of its constituent characters:

# Casting a string to a set
greeting = "hello"
greeting_set = set(greeting)
print(greeting_set)  # Outputs: {'e', 'h', 'l', 'o'}

In greeting_set, each character from the string greeting appears once, even though the letter ‘l’ was duplicated in the original string.

Practical Considerations

  1. Order and Sorting: Remember that the resulting set does not maintain the order from the original collection. Sets are unordered collections, and thus, the concept of order doesn’t apply to them.
  2. Hashability: Only hashable (immutable) objects can be part of a set. Lists and dictionaries cannot be set elements because they are mutable.
  3. Use for Uniqueness: Casting collections to sets is a common and efficient way to eliminate duplicate elements.
  4. Type of Elements: When casting from dictionaries, you only get the keys, not the values. If you need the values, you could use set(dict.values()).
  5. Performance: For large collections, especially when checking for membership frequently, converting a list or tuple to a set can significantly improve performance.

Working with Sets:

Working with sets in Python involves using a variety of methods and operations that can be performed on set objects. Sets are an incredibly powerful feature for certain tasks, particularly those involving uniqueness of elements and set theory operations. Below, we delve into some common functionalities provided by sets.

Adding Elements

To add a single element to a set, you use the add() method. This method takes a single argument, which is the element you want to add to the set. If the element is already in the set, the set doesn’t change because all elements in a set must be unique.

fruits = {'apple', 'banana'}
print(fruits)  # Outputs: {'apple', 'banana', 'cherry'}

In the above code, ‘cherry’ is added to the fruits set.

Updating a Set

If you need to add multiple elements to a set, you can use the update() method. It takes an iterable, such as another set, list, or tuple, and adds all elements to the set.

fruits = {'apple', 'banana'}
fruits.update(['cherry', 'date'])
print(fruits)  # Outputs: {'apple', 'banana', 'cherry', 'date'}

With update(), all elements in the iterable are added to the fruits set. If there are duplicates, they will not be added, as sets only hold unique elements.

Removing Elements

To remove an element from a set, you can use either the remove() or discard() method:

  • remove(elem) will remove elem from the set, and will raise a KeyError if elem is not present.
  • discard(elem) will also remove elem from the set, but will not raise an error if elem is not present.
fruits = {'apple', 'banana', 'cherry'}
print(fruits)  # Outputs: {'apple', 'cherry'}

fruits.discard('banana')  # No KeyError is raised

Popping Elements

The pop() method removes and returns an arbitrary element from the set. If the set is empty, calling pop() will raise a KeyError.

fruits = {'apple', 'banana', 'cherry'}
print(fruits.pop())  # Removes and returns an arbitrary element

Because sets are unordered, you cannot predict which element pop() will remove.

Clearing a Set

To remove all elements from a set, you can use the clear() method. This method removes all elements, effectively leaving you with an empty set.

fruits = {'apple', 'banana', 'cherry'}
print(fruits)  # Outputs: set()

Set Operations

Sets support typical mathematical set operations, such as union, intersection, difference, and symmetric difference.

  • Union (|): Combines all elements from both sets, omitting duplicates.
  • Intersection (&): Retrieves only the elements common to both sets.
  • Difference (-): Gets the elements that are in one set but not the other.
  • Symmetric Difference (^): Returns all elements from both sets, except those that are common to both.
a = {1, 2, 3}
b = {3, 4, 5}

# Union
print(a | b)  # Outputs: {1, 2, 3, 4, 5}

# Intersection
print(a & b)  # Outputs: {3}

# Difference
print(a - b)  # Outputs: {1, 2}

# Symmetric Difference
print(a ^ b)  # Outputs: {1, 2, 4, 5}

Set Methods for Set Operations

Python also provides named methods for these set operations that can be more readable:

  • union()
  • intersection()
  • difference()
  • symmetric_difference()

You can use these methods in a similar way to the operators:

# Union using method

# Intersection using method

# Difference using method

# Symmetric Difference using method

Set Comparisons

You can also check if a set is a subset, superset, or disjoint with respect to another set using methods like issubset(), issuperset(), and isdisjoint().

a = {1, 2}
b = {1, 2, 3}

# Subset check
print(a.issubset(b))  # Outputs: True

# Superset check
print(b.issuperset(a))  # Outputs: True

# Disjoint check
c = {4, 5}
print(a.isdisjoint(c))  # Outputs: True since a and c have no common elements

Frozenset: Immutable Sets

Python also offers an immutable version of sets called frozenset. It has the same characteristics as set but cannot be changed after it is created. This is useful for set elements and dictionary keys.

immutable_set = frozenset(['apple', 'banana', 'cherry'])

Working with sets in Python provides a robust way to handle data when uniqueness and set operations are required. The methods and operations available with sets are numerous, making them flexible and powerful tools in various scenarios, especially those involving complex collection manipulation.

Use Cases for Sets

Sets are particularly useful when you’re dealing with large datasets and you need to perform operations that involve uniqueness of elements.

Removing Duplicates

One common use case is removing duplicates from a collection:

# Removing duplicates from a list
data = [1, 2, 2, 3, 4, 4, 5]
unique_data = set(data)
print(list(unique_data))  # Converts the set back to a list

Membership Testing

Sets are optimized for membership testing, which can be more efficient than testing for membership in lists or tuples:

# Membership testing
inventory = set(["hammer", "wrench", "saw"])
print("hammer" in inventory)  # Outputs: True

Mathematical Set Operations

For tasks that require mathematical set operations, sets are the go-to data type:

# Finding common elements between two lists
science_students = {"John", "Alice", "James"}
math_students = {"Alice", "Dorothy", "James"}

# Using set intersection to find students who are in both classes
both_classes = science_students & math_students
print(both_classes)  # Outputs: {"Alice", "James"}

Caveats and Considerations

Immutability of Elements

As sets require elements to be hashable, mutable collections like lists or dictionaries cannot be set elements:

# Attempting to add a list to a set raises a TypeError
    my_set = {1, 2}
    my_set.add([3, 4])  # Raises TypeError
except TypeError:
    print("Cannot add list to a set.")

Order of Elements

The order of elements in a set is not guaranteed, and they may be returned in any order when iterating over a set or viewing its contents.


The set() function is an essential part of Python’s collection of data types and provides an efficient way to handle unique elements. From simplifying the removal of duplicates to performing complex set operations, sets are indispensable tools in a programmer’s toolkit. Understanding how to leverage sets can lead to cleaner, faster, and more efficient Python code.

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