Sets

Sets are used to store multiple items in a single variable.

Set is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Tuple, and Dictionary, all with different qualities and usage.

A set is a collection which is unordered, unchangeable*, and unindexed.

myset = {"apple", "banana", "cherry"}

Sets are written with curly brackets.

Unordered means that the items in a set do not have a defined order.

Set items can appear in a different order every time you use them, and cannot be referred to by index or key.

Set items are unchangeable, meaning that we cannot change the items after the set has been created.

Once a set is created, you cannot change its items, but you can remove items and add new items.

Sets cannot have two items with the same value.

Note: The values True and 1 are considered the same value in sets, and are treated as duplicates:

The values False and 0 are considered the same value in sets, and are treated as duplicates:

thisset = {"apple", "banana", "cherry"}
set1 = {"abc", 34, True, 40, "male"}
print(len(thisset))
print(type(set1))

output

3
<class 'set'>

constructor

thisset = set(("apple", "banana", "cherry")) # note the double round-brackets
print(thisset)

function

access

You cannot access items in a set by referring to an index or a key.

But you can loop through the set items using a for loop, or ask if a specified value is present in a set, by using the in keyword.

add

thisset = {"apple", "banana", "cherry"}
thisset.add("orange")
print(thisset)

tropical = {"pineapple", "mango", "papaya"}
thisset.update(tropical)
print(thisset)

output

{"apple","banana","cherry","orange"}
{"apple","banana","cherry","orange","pineapple","mango","papaya"}

The object in the update() method does not have to be a set, it can be any iterable object (tuples, lists, dictionaries etc.).

thisset = {"apple", "banana", "cherry"}
mylist = ["kiwi", "orange"]

thisset.update(mylist)
print(thisset)

remove

thisset = {"apple", "banana", "cherry"}
thisset.remove("banana")
print(thisset)

thisset.discard("banana")

Note

If the item to remove does not exist, remove() will raise an error.

If the item to remove does not exist, discard() will NOT raise an error.

You can also use the pop() method to remove an item, but this method will remove a random item, so you cannot be sure what item that gets removed.

The return value of the pop() method is the removed item.

thisset = {"apple", "banana", "cherry"}
x = thisset.pop()
print(x)
print(thisset)

thisset = {"apple", "banana", "cherry"}
thisset.clear()
print(thisset)
del thisset
print(thisset)

output

set()
NameError: name 'thisset' is not defined

loop

thisset = {"apple", "banana", "cherry"}
for x in thisset:
  print(x)

join

There are several ways to join two or more sets in Python.

You can use the union() method that returns a new set containing all items from both sets, or the update() method that inserts all the items from one set into another:

set1 = {"a", "b" , "c"}
set2 = {1, 2, 3}

set3 = set1.union(set2)
print(set3)

set1.update(set2)
print(set1)

注意

Note: Both union() and update() will exclude any duplicate items.

The intersection_update() method will keep only the items that are present in both sets.

The intersection() method will return a new set, that only contains the items that are present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

z = x.intersection(y)
print(z)

x.intersection_update(y)
print(x)

output

{"apple"}
{"apple"}

The symmetric_difference_update() method will keep only the elements that are NOT present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

z = x.symmetric_difference(y)
print(z)

x.symmetric_difference_update(y)
print(x)

output

{'google', 'banana', 'microsoft', 'cherry'}
{'google', 'banana', 'microsoft', 'cherry'}

The values True and 1 are considered the same value in sets, and are treated as duplicates.

function	description
add()	Adds an element to the set
clear()	Removes all the elements from the set
copy()	Returns a copy of the set
difference()	Returns a set containing the difference between two or more sets
difference_update()	Removes the items in this set that are also included in another, specified set
discard()	Remove the specified item
intersection()	Returns a set, that is the intersection of two other sets
intersection_update()	Removes the items in this set that are not present in other, specified set(s)
isdisjoint()	Returns whether two sets have a intersection or not
issubset()	Returns whether another set contains this set or not
issuperset()	Returns whether this set contains another set or not
pop()	Removes an element from the set
remove()	Removes the specified element
symmetric_difference()	Returns a set with the symmetric differences of two sets
symmetric_difference_update()	inserts the symmetric differences from this set and another
union()	Return a set containing the union of sets
update()	Update the set with the union of this set and others