# Sets in Python

A Set is an unordered collection data type that is iterable, mutable and has no duplicate elements. Python’s set class represents the mathematical notion of a set. The major advantage of using a set, as opposed to a list, is that it has a highly optimized method for checking whether a specific element is contained in the set. This is based on a data structure known as a hash table.

**Frozen Sets** Frozen sets are immutable objects that only support methods and operators that produce a result without affecting the frozen set or sets to which they are applied.

`# Python program to demonstrate differences ` `# between normal and frozen set ` ` ` `# Same as {"a", "b","c"} ` `normal_set ` `=` `set` `([` `"a"` `, ` `"b"` `,` `"c"` `]) ` ` ` `# Adding an element to normal set is fine ` `normal_set.add(` `"d"` `) ` ` ` `print` `(` `"Normal Set"` `) ` `print` `(normal_set) ` ` ` `# A frozen set ` `frozen_set ` `=` `frozenset` `([` `"e"` `, ` `"f"` `, ` `"g"` `]) ` ` ` `print` `(` `"Frozen Set"` `) ` `print` `(frozen_set) ` ` ` `# Uncommenting below line would cause error as ` `# we are trying to add element to a frozen set ` `# frozen_set.add("h") ` |

*chevron_right*

*filter_none*

Output:

Normal Set set(['a', 'c', 'b', 'd']) Frozen Set frozenset(['e', 'g', 'f'])

**Methods for Sets**

**1. add(x) Method: **Adds the item x to set if it is not already present in the set.

people = {"Jay", "Idrish", "Archil"} people.add("Daxit")

-> This will add Daxit in people set.

2. **union(s) Method**: Returns a union of two set.Using the ‘|’ operator between 2 sets is the same as writing set1.union(set2)

people = {"Jay", "Idrish", "Archil"} vampires = {"Karan", "Arjun"} population = people.union(vampires)

OR

population = people|vampires

-> Set population set will have components of both people and vampire

3.** intersect(s) Method: **Returns an intersection of two sets.The ‘&’ operator comes can also be used in this case.

victims = people.intersection(vampires)

-> Set victims will contain the common element of people and vampire

**4. difference(s) Method: **Returns a set containing all the elements of invoking set but not of the second set. We can use ‘-‘ operator here.

safe = people.difference(vampires)

OR

safe = people – vampires

-> Set safe will have all the elements that are in people but not vampire

**5. clear() Method: **Empties the whole set.

victims.clear()

-> Clears victim set

However there are two major pitfalls in Python sets:

- The set doesn’t maintain elements in any particular order.
- Only instances of immutable types can be added to a Python set.

**Operators for Sets**

Sets and frozen sets support the following operators:

key in s # containment check

key not in s # non-containment check

s1 == s2 # s1 is equivalent to s2

s1 != s2 # s1 is not equivalent to s2

s1 <= s2 # s1is subset of s2 s1 < s2 # s1 is proper subset of s2 s1 >= s2 # s1is superset of s2

s1 > s2 # s1 is proper superset of s2

s1 | s2 # the union of s1 and s2

s1 & s2 # the intersection of s1 and s2

s1 – s2 # the set of elements in s1 but not s2

s1 ˆ s2 # the set of elements in precisely one of s1 or s2

Code Snippet to illustrate all Set operations in Python

`# Python program to demonstrate working# of ` `# Set in Python ` ` ` `# Creating two sets ` `set1 ` `=` `set` `() ` `set2 ` `=` `set` `() ` ` ` `# Adding elements to set1 ` `for` `i ` `in` `range` `(` `1` `, ` `6` `): ` ` ` `set1.add(i) ` ` ` `# Adding elements to set2 ` `for` `i ` `in` `range` `(` `3` `, ` `8` `): ` ` ` `set2.add(i) ` ` ` `print` `(` `"Set1 = "` `, set1) ` `print` `(` `"Set2 = "` `, set2) ` `print` `(` `"\n"` `) ` ` ` `# Union of set1 and set2 ` `set3 ` `=` `set1 | set2` `# set1.union(set2) ` `print` `(` `"Union of Set1 & Set2: Set3 = "` `, set3) ` ` ` `# Intersection of set1 and set2 ` `set4 ` `=` `set1 & set2` `# set1.intersection(set2) ` `print` `(` `"Intersection of Set1 & Set2: Set4 = "` `, set4) ` `print` `(` `"\n"` `) ` ` ` `# Checking relation between set3 and set4 ` `if` `set3 > set4: ` `# set3.issuperset(set4) ` ` ` `print` `(` `"Set3 is superset of Set4"` `) ` `elif` `set3 < set4: ` `# set3.issubset(set4) ` ` ` `print` `(` `"Set3 is subset of Set4"` `) ` `else` `: ` `# set3 == set4 ` ` ` `print` `(` `"Set3 is same as Set4"` `) ` ` ` `# displaying relation between set4 and set3 ` `if` `set4 < set3: ` `# set4.issubset(set3) ` ` ` `print` `(` `"Set4 is subset of Set3"` `) ` ` ` `print` `(` `"\n"` `) ` ` ` `# difference between set3 and set4 ` `set5 ` `=` `set3 ` `-` `set4 ` `print` `(` `"Elements in Set3 and not in Set4: Set5 = "` `, set5) ` `print` `(` `"\n"` `) ` ` ` `# checkv if set4 and set5 are disjoint sets ` `if` `set4.isdisjoint(set5): ` ` ` `print` `(` `"Set4 and Set5 have nothing in common\n"` `) ` ` ` `# Removing all the values of set5 ` `set5.clear() ` ` ` `print` `(` `"After applying clear on sets Set5: "` `) ` `print` `(` `"Set5 = "` `, set5) ` |

*chevron_right*

*filter_none*

Output:

('Set1 = ', set([1, 2, 3, 4, 5])) ('Set2 = ', set([3, 4, 5, 6, 7])) ('Union of Set1 & Set2: Set3 = ', set([1, 2, 3, 4, 5, 6, 7])) ('Intersection of Set1 & Set2: Set4 = ', set([3, 4, 5])) Set3 is superset of Set4 Set4 is subset of Set3 ('Elements in Set3 and not in Set4: Set5 = ', set([1, 2, 6, 7])) Set4 and Set5 have nothing in common After applying clear on sets Set5: ('Set5 = ', set([]))

**Recent articles on Python Set.**

This article is contributed by **Jay Patel.** If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

## Recommended Posts:

- Python Sets
- Python | remove() and discard() in Sets
- Output of Python Programs | Set 24 (Sets)
- Python | sympy.sets.Ropen() method
- Python Set | Pairs of complete strings in two sets
- Python | sympy.sets.open() method
- Python | sympy.sets.Lopen() method
- Python program to find common elements in three lists using sets
- Python program to count number of vowels using sets in given string
- Cartesian Product of any number of sets
- Python | Convert list to Python array
- Reading Python File-Like Objects from C | Python
- Python | Merge Python key values to list
- Important differences between Python 2.x and Python 3.x with examples
- Python | Index of Non-Zero elements in Python list