Python Sets
Set
A set is a collection which is unordered and unindexed. In Python, sets are written with curly brackets.
Note: Sets are unordered, so you cannot be sure in which order the items will appear.
Access Items
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.
Example
Loop through the set, and print the values:
thisset = {"apple", "banana", "cherry"}
for x in thisset:
print(x)
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Example
Check if "banana" is present in the set:
thisset = {"apple", "banana", "cherry"}
print("banana"
in thisset)
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Change Items
Once a set is created, you cannot change its items, but you can add new items.
Add Items
To add one item to a set use the add()
method.
To add more than one item to a set use the update()
method.
Example
Add an item to a set, using the add()
method:
thisset = {"apple", "banana", "cherry"}
thisset.add("orange")
print(thisset)
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Example
Add multiple items to a set, using the update()
method:
thisset = {"apple", "banana", "cherry"}
thisset.update(["orange",
"mango", "grapes"])
print(thisset)
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Get the Length of a Set
To determine how many items a set has, use the len() method.
Example
Get the number of items in a set:
thisset = {"apple", "banana", "cherry"}
print(len(thisset))
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Remove Item
To remove an item in a set, use the remove(), or the discard() method.
Example
Remove "banana" by using the remove()
method:
thisset = {"apple", "banana", "cherry"}
thisset.remove("banana")
print(thisset)
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Note: If the item to remove does not exist, remove() will raise an error.
Example
Remove "banana" by using the discard()
method:
thisset = {"apple", "banana", "cherry"}
thisset.discard("banana")
print(thisset)
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Note: 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 the last item. Remember that sets
are unordered, so you will not know what item that gets removed.
The return value of the pop() method is the
removed item.
Example
Remove the last item by using the pop()
method:
thisset = {"apple", "banana", "cherry"}
x =
thisset.pop()
print(x)
print(thisset)
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Note: Sets are unordered, so when using the pop() method,
you will not know which item that gets removed.
Example
The clear()
method empties the set:
thisset = {"apple", "banana", "cherry"}
thisset.clear()
print(thisset)
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Example
The del keyword will delete the set
completely:
thisset = {"apple", "banana", "cherry"}
del
thisset
print(thisset)
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Join Two Sets
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:
Example
The union() method returns a new set with all items from both sets:
set1 = {"a", "b" , "c"}
set2 = {1, 2, 3}
set3 = set1.union(set2)
print(set3)
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Example
The update() method inserts the items in set2 into set1:
set1 = {"a", "b" , "c"}
set2 = {1, 2, 3}
set1.update(set2)
print(set1)
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Note: Both union() and update()
will exclude any duplicate items.
There are other methods that joins two sets and keeps ONLY the duplicates, or NEVER the duplicates, check the full list of set methods in the bottom of this page.
The set() Constructor
It is also possible to use the set() constructor to make a set.
Example
Using the set() constructor to make a set:
thisset = set(("apple", "banana", "cherry")) # note the double round-brackets
print(thisset)
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Set Methods
Python has a set of built-in methods that you can use on sets.
| Method | 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 |
