Set
A set is a collection which is unordered and unindexed. In Python sets are written with curly brackets.
Example
Create a Set:
thisset = {"apple", "banana", "cherry"}
print(thisset)
Show Python »C:\Users\My Name>python demo_set.py
{'cherry', 'banana', 'apple'}
Note: Sets are unordered, so the items will appear in a random order.
Access Items
You cannot access items in a set by referring to an index, since sets are unordered the items has no index.
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 thein keyword.Example
Loop through the set, and print the values:
thisset = {"apple", "banana", "cherry"}
for x in thisset:
print(x)
Show Python »C:\Users\My Name>python demo_set_loop.py
cherry
banana
apple
Example
Check if "banana" is present in the set:
thisset = {"apple", "banana", "cherry"}
print("banana" in thisset)
Show Python »C:\Users\My Name>python demo_set_in.py
True
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)
Show Python »C:\Users\My Name>python demo_set_add.py
{'apple', 'banana', 'orange', 'cherry'}
Example
Add multiple items to a set, using the
update() method:
thisset = {"apple", "banana", "cherry"}
thisset.update(["orange", "mango", "grapes"])
print(thisset)
Show Python »C:\Users\My Name>python demo_set_update.py
{'orange', 'mango', 'cherry', 'grapes', 'banana', 'apple'}
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))
Show Python »C:\Users\My Name>python demo_set_length.py
3
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)
Show Python »C:\Users\My Name>python demo_set_remove.py
{'cherry', 'apple'}
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)
Show Python »C:\Users\My Name>python demo_set_discard.py
{'cherry', 'apple'}
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)
Show Python »C:\Users\My Name>python demo_set_pop.py
cherry
{'apple', 'banana'}
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)
Show Python »C:\Users\My Name>python demo_set_clear.py
set()
Example
The
del keyword will delete the set completely:
thisset = {"apple", "banana", "cherry"}
del thisset
print(thisset)
Show Python »C:\Users\My Name>python demo_set_del.py
Traceback (most recent call last):
File "demo_set_del.py", line 5, in <module>
print(thisset) #this will raise an error because the set no longer exists
NameError: name 'thisset' is not defined
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-bracketsprint(thisset)
Show Python »C:\Users\My Name>python demo_set2.py
{'banana', 'cherry', 'apple'}
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 |
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