COLLECTIONS

LISTS

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

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

Lists are created using square brackets:

Example

Create a List:

thislist = ["apple", "banana", "cherry"]

print(thislist)

Try it Yourself »

List Items

List items are ordered, changeable, and allow duplicate values.

List items are indexed, the first item has index [0], the second item has index [1] etc.

Ordered

When we say that lists are ordered, it means that the items have a defined order, and that order will not change.

If you add new items to a list, the new items will be placed at the end of the list.

Note: There are some list methods that will change the order, but in general: the order of the items will not change.

Changeable

The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

Allow Duplicates

Since lists are indexed, lists can have items with the same value:

Example

Lists allow duplicate values:

thislist = ["apple", "banana", "cherry", "apple", "cherry"]

print(thislist)

Try it Yourself »

List Length

To determine how many items a list has, use the len() function:

Example

Print the number of items in the list:

thislist = ["apple", "banana", "cherry"]

print(len(thislist))

Try it Yourself »

List Items - Data Types

List items can be of any data type:

Example

String, int and boolean data types:

list1 = ["apple", "banana", "cherry"]

list2 = [1, 5, 7, 9, 3]

list3 = [True, False, False]

Try it Yourself »

A list can contain different data types:

Example

A list with strings, integers and boolean values:

list1 = ["abc", 34, True, 40, "male"]

Try it Yourself »

type()

From Python's perspective, lists are defined as objects with the data type 'list':

<class 'list'>

Example

What is the data type of a list?

mylist = ["apple", "banana", "cherry"]

print(type(mylist))

Try it Yourself »

The list() Constructor

It is also possible to use the list() constructor when creating a new list.

Example

Using the list() constructor to make a List:

thislist = list(("apple", "banana", "cherry")) # note the double round-brackets

print(thislist)

Try it Yourself »

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.

• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

• Set is a collection which is unordered and unindexed. No duplicate members.

• Dictionary is a collection which is ordered* and changeable. No duplicate members.

*As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

Access Items

List items are indexed and you can access them by referring to the index number:

Example

Print the second item of the list:

thislist = ["apple", "banana", "cherry"]

print(thislist[1])

Try it Yourself »

Note: The first item has index 0.

Negative Indexing

Negative indexing means start from the end

-1 refers to the last item, -2 refers to the second last item etc.

Example

Print the last item of the list:

thislist = ["apple", "banana", "cherry"]

print(thislist[-1])

Try it Yourself »

Range of Indexes

You can specify a range of indexes by specifying where to start and where to end the range.

When specifying a range, the return value will be a new list with the specified items.

Example

Return the third, fourth, and fifth item:

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

print(thislist[2:5])

Try it Yourself »

Note: The search will start at index 2 (included) and end at index 5 (not included).

Remember that the first item has index 0.

By leaving out the start value, the range will start at the first item:

Example

This example returns the items from the beginning to, but NOT including, "kiwi":

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

print(thislist[:4])

Try it Yourself »

By leaving out the end value, the range will go on to the end of the list:

Example

This example returns the items from "cherry" to the end:

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

print(thislist[2:])

Try it Yourself »

Range of Negative Indexes

Specify negative indexes if you want to start the search from the end of the list:

Example

This example returns the items from "orange" (-4) to, but NOT including "mango" (-1):

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

print(thislist[-4:-1])

Try it Yourself »

Check if Item Exists

To determine if a specified item is present in a list use the in keyword:

Example

Check if "apple" is present in the list:

thislist = ["apple", "banana", "cherry"]

if "apple" in thislist:

print("Yes, 'apple' is in the fruits list")

Change Item Value

To change the value of a specific item, refer to the index number:

Example

Change the second item:

thislist = ["apple", "banana", "cherry"]

thislist[1] = "blackcurrant"

print(thislist)

Try it Yourself »

Change a Range of Item Values

To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:

Example

Change the values "banana" and "cherry" with the values "blackcurrant" and "watermelon":

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]

thislist[1:3] = ["blackcurrant", "watermelon"]

print(thislist)

Try it Yourself »

If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

Example

Change the second value by replacing it with two new values:

thislist = ["apple", "banana", "cherry"]

thislist[1:2] = ["blackcurrant", "watermelon"]

print(thislist)

Try it Yourself »

Note: The length of the list will change when the number of items inserted does not match the number of items replaced.

If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

Example

Change the second and third value by replacing it with one value:

thislist = ["apple", "banana", "cherry"]

thislist[1:3] = ["watermelon"]

print(thislist)

Try it Yourself »

Insert Items

To insert a new list item, without replacing any of the existing values, we can use the insert() method.

The insert() method inserts an item at the specified index:

Example

Insert "watermelon" as the third item:

thislist = ["apple", "banana", "cherry"]

thislist.insert(2, "watermelon")

print(thislist)

Try it Yourself »

Append Items

To add an item to the end of the list, use the append() method:

Example

Using the append() method to append an item:

thislist = ["apple", "banana", "cherry"]

thislist.append("orange")

print(thislist)

Try it Yourself »

Insert Items

To insert a list item at a specified index, use the insert() method.

The insert() method inserts an item at the specified index:

Example

Insert an item as the second position:

thislist = ["apple", "banana", "cherry"]

thislist.insert(1, "orange")

print(thislist)

Try it Yourself »

Note: As a result of the examples above, the lists will now contain 4 items.

Extend List

To append elements from another list to the current list, use the extend() method.

Example

Add the elements of tropical to thislist:

thislist = ["apple", "banana", "cherry"]

tropical = ["mango", "pineapple", "papaya"]

thislist.extend(tropical)

print(thislist)

Try it Yourself »

The elements will be added to the end of the list.

Add Any Iterable

The extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).

Example

Add elements of a tuple to a list:

thislist = ["apple", "banana", "cherry"]

thistuple = ("kiwi", "orange")

thislist.extend(thistuple)

print(thislist)

Try it Yourself »

Remove Specified Item

The remove() method removes the specified item.

Example

Remove "banana":

thislist = ["apple", "banana", "cherry"]

thislist.remove("banana")

print(thislist)

Try it Yourself »

Remove Specified Index

The pop() method removes the specified index.

Example

Remove the second item:

thislist = ["apple", "banana", "cherry"]

thislist.pop(1)

print(thislist)

Try it Yourself »

If you do not specify the index, the pop() method removes the last item.

Example

Remove the last item:

thislist = ["apple", "banana", "cherry"]

thislist.pop()

print(thislist)

Try it Yourself »

The del keyword also removes the specified index:

Example

Remove the first item:

thislist = ["apple", "banana", "cherry"]

del thislist[0]

print(thislist)

Try it Yourself »

The del keyword can also delete the list completely.

Example

Delete the entire list:

thislist = ["apple", "banana", "cherry"]

del thislist

Try it Yourself »

Clear the List

The clear() method empties the list.

The list still remains, but it has no content.

Example

Clear the list content:

thislist = ["apple", "banana", "cherry"]

thislist.clear()

print(thislist)

Loop Through a List

You can loop through the list items by using a for loop:

Example

Print all items in the list, one by one:

thislist = ["apple", "banana", "cherry"]

for x in thislist:

print(x)

Try it Yourself »

Learn more about for loops in our Python For Loops Chapter.

Loop Through the Index Numbers

You can also loop through the list items by referring to their index number.

Use the range() and len() functions to create a suitable iterable.

Example

Print all items by referring to their index number:

thislist = ["apple", "banana", "cherry"]

for i in range(len(thislist)):

print(thislist[i])

Try it Yourself »

The iterable created in the example above is [0, 1, 2].

Using a While Loop

You can loop through the list items by using a while loop.

Use the len() function to determine the length of the list, then start at 0 and loop your way through the list items by refering to their indexes.

Remember to increase the index by 1 after each iteration.

Example

Print all items, using a while loop to go through all the index numbers

thislist = ["apple", "banana", "cherry"]

i = 0

while i < len(thislist):

print(thislist[i])

i = i + 1

Try it Yourself »

Learn more about while loops in our Python While Loops Chapter.

Looping Using List Comprehension

List Comprehension offers the shortest syntax for looping through lists:

Example

A short hand for loop that will print all items in a list:

thislist = ["apple", "banana", "cherry"]

[print(x) for x in thislist]

List Comprehension

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.

Example:

Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.

Without list comprehension you will have to write a for statement with a conditional test inside:

Example

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = []

for x in fruits:

if "a" in x:

newlist.append(x)

print(newlist)

Try it Yourself »

With list comprehension you can do all that with only one line of code:

Example

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if "a" in x]

print(newlist)

Try it Yourself »

The Syntax

newlist = [expression for item in iterable if condition == True]

The return value is a new list, leaving the old list unchanged.

Condition

The condition is like a filter that only accepts the items that valuate to True.

Example

Only accept items that are not "apple":

newlist = [x for x in fruits if x != "apple"]

Try it Yourself »

The condition if x != "apple" will return True for all elements other than "apple", making the new list contain all fruits except "apple".

The condition is optional and can be omitted:

Example

With no if statement:

newlist = [x for x in fruits]

Try it Yourself »

Iterable

The iterable can be any iterable object, like a list, tuple, set etc.

Example

You can use the range() function to create an iterable:

newlist = [x for x in range(10)]

Try it Yourself »

Same example, but with a condition:

Example

Accept only numbers lower than 5:

newlist = [x for x in range(10) if x < 5]

Try it Yourself »

Expression

The expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list:

Example

Set the values in the new list to upper case:

newlist = [x.upper() for x in fruits]

Try it Yourself »

You can set the outcome to whatever you like:

Example

Set all values in the new list to 'hello':

newlist = ['hello' for x in fruits]

Try it Yourself »

The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome:

Example

Return "orange" instead of "banana":

newlist = [x if x != "banana" else "orange" for x in fruits]

Try it Yourself »

The expression in the example above says:

"Return the item if it is not banana, if it is banana return orange".

Sort List Alphanumerically

List objects have a sort() method that will sort the list alphanumerically, ascending, by default:

Example

Sort the list alphabetically:

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]

thislist.sort()

print(thislist)

Try it Yourself »

Example

Sort the list numerically:

thislist = [100, 50, 65, 82, 23]

thislist.sort()

print(thislist)

Try it Yourself »

Sort Descending

To sort descending, use the keyword argument reverse = True:

Example

Sort the list descending:

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]

thislist.sort(reverse = True)

print(thislist)

Try it Yourself »

Example

Sort the list descending:

thislist = [100, 50, 65, 82, 23]

thislist.sort(reverse = True)

print(thislist)

Try it Yourself »

Customize Sort Function

You can also customize your own function by using the keyword argument key = function.

The function will return a number that will be used to sort the list (the lowest number first):

Example

Sort the list based on how close the number is to 50:

def myfunc(n):

return abs(n - 50)

thislist = [100, 50, 65, 82, 23]

thislist.sort(key = myfunc)

print(thislist)

Try it Yourself »

Case Insensitive Sort

By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters:

Example

Case sensitive sorting can give an unexpected result:

thislist = ["banana", "Orange", "Kiwi", "cherry"]

thislist.sort()

print(thislist)

Try it Yourself »

Luckily we can use built-in functions as key functions when sorting a list.

So if you want a case-insensitive sort function, use str.lower as a key function:

Example

Perform a case-insensitive sort of the list:

thislist = ["banana", "Orange", "Kiwi", "cherry"]

thislist.sort(key = str.lower)

print(thislist)

Try it Yourself »

Reverse Order

What if you want to reverse the order of a list, regardless of the alphabet?

The reverse() method reverses the current sorting order of the elements.

Example

Reverse the order of the list items:

thislist = ["banana", "Orange", "Kiwi", "cherry"]

thislist.reverse()

print(thislist)

Copy a List

You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.

There are ways to make a copy, one way is to use the built-in List method copy().

Example

Make a copy of a list with the copy() method:

thislist = ["apple", "banana", "cherry"]

mylist = thislist.copy()

print(mylist)

Try it Yourself »

Another way to make a copy is to use the built-in method list().

Example

Make a copy of a list with the list() method:

thislist = ["apple", "banana", "cherry"]

mylist = list(thislist)

print(mylist)

Join Two Lists

There are several ways to join, or concatenate, two or more lists in Python.

One of the easiest ways are by using the + operator.

Example

Join two list:

list1 = ["a", "b", "c"]

list2 = [1, 2, 3]

list3 = list1 + list2

print(list3)

Try it Yourself »

Another way to join two lists are by appending all the items from list2 into list1, one by one:

Example

Append list2 into list1:

list1 = ["a", "b" , "c"]

list2 = [1, 2, 3]

for x in list2:

list1.append(x)

print(list1)

Try it Yourself »

Or you can use the extend() method, which purpose is to add elements from one list to another list:

Example

Use the extend() method to add list2 at the end of list1:

list1 = ["a", "b" , "c"]

list2 = [1, 2, 3]

list1.extend(list2)

print(list1)

List Methods

Python has a set of built-in methods that you can use on lists.

Method

Description

append()

Adds an element at the end of the list

clear()

Removes all the elements from the list

copy()

Returns a copy of the list

count()

Returns the number of elements with the specified value

extend()

Add the elements of a list (or any iterable), to the end of the current list

index()

Returns the index of the first element with the specified value

insert()

Adds an element at the specified position

pop()

Removes the element at the specified position

remove()

Removes the item with the specified value

reverse()

Reverses the order of the list

sort()

Sorts the list

TUPLE

Tuple

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

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

A tuple is a collection which is ordered and unchangeable.

Tuples are written with round brackets.

Example

Create a Tuple:

thistuple = ("apple", "banana", "cherry")

print(thistuple)

Try it Yourself »

Tuple Items

Tuple items are ordered, unchangeable, and allow duplicate values.

Tuple items are indexed, the first item has index [0], the second item has index [1] etc.

Ordered

When we say that tuples are ordered, it means that the items have a defined order, and that order will not change.

Unchangeable

Tuples are unchangeable, meaning that we cannot change, add or remove items after the tuple has been created.

Allow Duplicates

Since tuple are indexed, tuples can have items with the same value:

Example

Tuples allow duplicate values:

thistuple = ("apple", "banana", "cherry", "apple", "cherry")

print(thistuple)

Try it Yourself »

Tuple Length

To determine how many items a tuple has, use the len() function:

Example

Print the number of items in the tuple:

thistuple = ("apple", "banana", "cherry")

print(len(thistuple))

Try it Yourself »

Create Tuple With One Item

To create a tuple with only one item, you have to add a comma after the item, otherwise Python will not recognize it as a tuple.

Example

One item tuple, remember the commma:

thistuple = ("apple",)

print(type(thistuple))

#NOT a tuple

thistuple = ("apple")

print(type(thistuple))

Try it Yourself »

Tuple Items - Data Types

Tuple items can be of any data type:

Example

String, int and boolean data types:

tuple1 = ("apple", "banana", "cherry")

tuple2 = (1, 5, 7, 9, 3)

tuple3 = (True, False, False)

Try it Yourself »

A tuple can contain different data types:

Example

A tuple with strings, integers and boolean values:

tuple1 = ("abc", 34, True, 40, "male")

Try it Yourself »

type()

From Python's perspective, tuples are defined as objects with the data type 'tuple':

<class 'tuple'>

Example

What is the data type of a tuple?

mytuple = ("apple", "banana", "cherry")

print(type(mytuple))

Try it Yourself »

The tuple() Constructor

It is also possible to use the tuple() constructor to make a tuple.

Example

Using the tuple() method to make a tuple:

thistuple = tuple(("apple", "banana", "cherry")) # note the double round-brackets

print(thistuple)

Try it Yourself »

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.

• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

• Set is a collection which is unordered and unindexed. No duplicate members.

• Dictionary is a collection which is ordered* and changeable. No duplicate members.

*As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

Access Tuple Items

You can access tuple items by referring to the index number, inside square brackets:

Example

Print the second item in the tuple:

thistuple = ("apple", "banana", "cherry")

print(thistuple[1])

Try it Yourself »

Note: The first item has index 0.

Negative Indexing

Negative indexing means start from the end.

-1 refers to the last item, -2 refers to the second last item etc.

Example

Print the last item of the tuple:

thistuple = ("apple", "banana", "cherry")

print(thistuple[-1])

Try it Yourself »

Range of Indexes

You can specify a range of indexes by specifying where to start and where to end the range.

When specifying a range, the return value will be a new tuple with the specified items.

Example

Return the third, fourth, and fifth item:

thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")

print(thistuple[2:5])

Try it Yourself »

Note: The search will start at index 2 (included) and end at index 5 (not included).

Remember that the first item has index 0.

By leaving out the start value, the range will start at the first item:

Example

This example returns the items from the beginning to, but NOT included, "kiwi":

thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")

print(thistuple[:4])

Try it Yourself »

By leaving out the end value, the range will go on to the end of the list:

Example

This example returns the items from "cherry" and to the end:

thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")

print(thistuple[2:])

Try it Yourself »

Range of Negative Indexes

Specify negative indexes if you want to start the search from the end of the tuple:

Example

This example returns the items from index -4 (included) to index -1 (excluded)

thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")

print(thistuple[-4:-1])

Try it Yourself »

Check if Item Exists

To determine if a specified item is present in a tuple use the in keyword:

Example

Check if "apple" is present in the tuple:

thistuple = ("apple", "banana", "cherry")

if "apple" in thistuple:

print("Yes, 'apple' is in the fruits tuple")

Tuples are unchangeable, meaing that you cannot change, add, or remove items once the tuple is created.

But there are some workarounds.

Change Tuple Values

Once a tuple is created, you cannot change its values. Tuples are unchangeable, or immutable as it also is called.

But there is a workaround. You can convert the tuple into a list, change the list, and convert the list back into a tuple.

Example

Convert the tuple into a list to be able to change it:

x = ("apple", "banana", "cherry")

y = list(x)

y[1] = "kiwi"

x = tuple(y)

print(x)

Try it Yourself »

Add Items

Once a tuple is created, you cannot add items to it.

Example

You cannot add items to a tuple:

thistuple = ("apple", "banana", "cherry")

thistuple.append("orange") # This will raise an error

print(thistuple)

Try it Yourself »

Just like the workaround for changing a tuple, you can convert it into a list, add your item(s), and convert it back into a tuple.

Example

Convert the tuple into a list, add "orange", and convert it back into a tuple:

thistuple = ("apple", "banana", "cherry")

y = list(thistuple)

y.append("orange")

thistuple = tuple(y)

Try it Yourself »

Remove Items

Note: You cannot remove items in a tuple.

Tuples are unchangeable, so you cannot remove items from it, but you can use the same workaround as we used for changing and adding tuple items:

Example

Convert the tuple into a list, remove "apple", and convert it back into a tuple:

thistuple = ("apple", "banana", "cherry")

y = list(thistuple)

y.remove("apple")

thistuple = tuple(y)

Try it Yourself »

Or you can delete the tuple completely:

Example

The del keyword can delete the tuple completely:

thistuple = ("apple", "banana", "cherry")

del thistuple

print(thistuple) #this will raise an error because the tuple no longer exists

Try it Yourself »

Unpacking a Tuple

When we create a tuple, we normally assign values to it. This is called "packing" a tuple:

Example

Packing a tuple:

fruits = ("apple", "banana", "cherry")

Try it Yourself »

But, in Python, we are also allowed to extract the values back into variables. This is called "unpacking":

Example

Unpacking a tuple:

fruits = ("apple", "banana", "cherry")

(green, yellow, red) = fruits

print(green)

print(yellow)

print(red)

Try it Yourself »

Note: The number of variables must match the number of values in the tuple, if not, you must use an asterix to collect the remaining values as a list.

Using Asterisk*

If the number of variables is less than the number of values, you can add an * to the variable name and the values will be assigned to the variable as a list:

Example

Assign the rest of the values as a list called "red":

fruits = ("apple", "banana", "cherry", "strawberry", "raspberry")

(green, yellow, *red) = fruits

print(green)

print(yellow)

print(red)

Try it Yourself »

If the asterix is added to another variable name than the last, Python will assign values to the variable until the number of values left matches the number of variables left.

Example

Add a list of values the "tropic" variable:

fruits = ("apple", "mango", "papaya", "pineapple", "cherry")

(green, *tropic, red) = fruits

print(green)

print(tropic)

print(red)

Loop Through a Tuple

You can loop through the tuple items by using a for loop.

Example

Iterate through the items and print the values:

thistuple = ("apple", "banana", "cherry")

for x in thistuple:

print(x)

Try it Yourself »

Learn more about for loops in our Python For Loops Chapter.

Loop Through the Index Numbers

You can also loop through the tuple items by referring to their index number.

Use the range() and len() functions to create a suitable iterable.

Example

Print all items by referring to their index number:

thistuple = ("apple", "banana", "cherry")

for i in range(len(thistuple)):

print(thistuple[i])

Try it Yourself »

Using a While Loop

You can loop through the list items by using a while loop.

Use the len() function to determine the length of the tuple, then start at 0 and loop your way through the tuple items by refering to their indexes.

Remember to increase the index by 1 after each iteration.

Example

Print all items, using a while loop to go through all the index numbers:

thistuple = ("apple", "banana", "cherry")

i = 0

while i < len(thistuple):

print(thistuple[i])

i = i + 1

Join Two Tuples

To join two or more tuples you can use the + operator:

Example

Join two tuples:

tuple1 = ("a", "b" , "c")

tuple2 = (1, 2, 3)

tuple3 = tuple1 + tuple2

print(tuple3)

Try it Yourself »

Multiply Tuples

If you want to multiply the content of a tuple a given number of times, you can use the * operator:

Example

Multiply the fruits tuple by 2:

fruits = ("apple", "banana", "cherry")

mytuple = fruits * 2

print(mytuple)

Tuple Methods

Python has two built-in methods that you can use on tuples.

Method

Description

count()

Returns the number of times a specified value occurs in a tuple

index()

Searches the tuple for a specified value and returns the position of where it was found

❮ PreviousNex

SETS

Set

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 both unordered and unindexed.

Sets are written with curly brackets.

Example

Create a Set:

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

print(thisset)

Try it Yourself »

Note: Sets are unordered, so you cannot be sure in which order the items will appear.

Set Items

Set items are unordered, unchangeable, and do not allow duplicate values.

Unordered

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.

Unchangeable

Sets 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 add new items.

Duplicates Not Allowed

Sets cannot have two items with the same value.

Example

Duplicate values will be ignored:

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

print(thisset)

Try it Yourself »

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))

Try it Yourself »

Set Items - Data Types

Set items can be of any data type:

Example

String, int and boolean data types:

set1 = {"apple", "banana", "cherry"}

set2 = {1, 5, 7, 9, 3}

set3 = {True, False, False}

Try it Yourself »

A set can contain different data types:

Example

A set with strings, integers and boolean values:

set1 = {"abc", 34, True, 40, "male"}

Try it Yourself »

type()

From Python's perspective, sets are defined as objects with the data type 'set':

<class 'set'>

Example

What is the data type of a set?

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

print(type(myset))

Try it Yourself »

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)

Try it Yourself »

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.

• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

• Set is a collection which is unordered and unindexed. No duplicate members.

• Dictionary is a collection which is ordered* and changeable. No duplicate members.

*As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

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)

Try it Yourself »

Example

Check if "banana" is present in the set:

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

print("banana" in thisset)

Try it Yourself »

Change Items

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

Add Items

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

To add one item to a set use the add() method.

Example

Add an item to a set, using the add() method:

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

thisset.add("orange")

print(thisset)

Try it Yourself »

Add Sets

To add items from another set into the current set, use the update() method.

Example

Add elements from tropical and thisset into newset:

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

tropical = {"pineapple", "mango", "papaya"}

thisset.update(tropical)

print(thisset)

Try it Yourself »

Add Any Iterable

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

Example

Add elements of a list to at set:

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

mylist = ["kiwi", "orange"]

thisset.update(mylist)

print(thisset)

Add Items

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

To add one item to a set use the add() method.

Example

Add an item to a set, using the add() method:

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

thisset.add("orange")

print(thisset)

Try it Yourself »

Add Sets

To add items from another set into the current set, use the update() method.

Example

Add elements from tropical and thisset into newset:

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

tropical = {"pineapple", "mango", "papaya"}

thisset.update(tropical)

print(thisset)

Try it Yourself »

Add Any Iterable

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

Example

Add elements of a list to at set:

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

mylist = ["kiwi", "orange"]

thisset.update(mylist)

print(thisset)

Loop Items

You can loop through the set items by using a for loop:

Example

Loop through the set, and print the values:

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

for x in thisset:

print(x)

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)

Try it Yourself »

Example

The update() method inserts the items in set2 into set1:

set1 = {"a", "b" , "c"}

set2 = {1, 2, 3}

set1.update(set2)

print(set1)

Try it Yourself »

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

Keep ONLY the Duplicates

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

Example

Keep the items that exist in both set x, and set y:

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

y = {"google", "microsoft", "apple"}

x.intersection_update(y)

print(x)

Try it Yourself »

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

Example

Return a set that contains the items that exist in both set x, and set y:

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

y = {"google", "microsoft", "apple"}

z = x.intersection(y)

print(z)

Try it Yourself »

Keep All, But NOT the Duplicates

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

Example

Keep the items that are not present in both sets:

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

y = {"google", "microsoft", "apple"}

x.symmetric_difference_update(y)

print(x)

Try it Yourself »

The symmetric_difference() method will return a new set, that contains only the elements that are NOT present in both sets.

Example

Return a set that contains all items from both sets, except items that are present in both:

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

y = {"google", "microsoft", "apple"}

z = x.symmetric_difference(y)

print(z)

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

DICTIONARIES

Dictionary

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered*, changeable and does not allow duplicates.

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

Dictionaries are written with curly brackets, and have keys and values:

Example

Create and print a dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(thisdict)

Try it Yourself »

Dictionary Items

Dictionary items are ordered, changeable, and does not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

Example

Print the "brand" value of the dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(thisdict["brand"])

Try it Yourself »

Ordered or Unordered?

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.

Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

Example

Duplicate values will overwrite existing values:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964,

"year": 2020

}

print(thisdict)

Try it Yourself »

Dictionary Length

To determine how many items a dictionary has, use the len() function:

Example

Print the number of items in the dictionary:

print(len(thisdict))

Try it Yourself »

Dictionary Items - Data Types

The values in dictionary items can be of any data type:

Example

String, int, boolean, and list data types:

thisdict = {

"brand": "Ford",

"electric": False,

"year": 1964,

"colors": ["red", "white", "blue"]

}

Try it Yourself »

type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

<class 'dict'>

Example

Print the data type of a dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(type(thisdict))

Try it Yourself »

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.

• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

• Set is a collection which is unordered and unindexed. No duplicate members.

• Dictionary is a collection which is unordered and changeable. No duplicate members.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

Example

Get the value of the "model" key:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = thisdict["model"]

Try it Yourself »

There is also a method called get() that will give you the same result:

Example

Get the value of the "model" key:

x = thisdict.get("model")

Try it Yourself »

Get Keys

The keys() method will return a list of all the keys in the dictionary.

Example

Get a list of the keys:

x = thisdict.keys()

Try it Yourself »

The list of the keys is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the keys list.

Example

Add a new item to the original dictionary, and see that the keys list gets updated as well:

car = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = car.keys()

print(x) #before the change

car["color"] = "white"

print(x) #after the change

Try it Yourself »

Get Values

The values() method will return a list of all the values in the dictionary.

Example

Get a list of the values:

x = thisdict.values()

Try it Yourself »

The list of the values is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the values list.

Example

Make a change in the original dictionary, and see that the values list gets updated as well:

car = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = car.values()

print(x) #before the change

car["year"] = 2020

print(x) #after the change

Try it Yourself »

Example

Add a new item to the original dictionary, and see that the values list gets updated as well:

car = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = car.values()

print(x) #before the change

car["color"] = "red"

print(x) #after the change

Try it Yourself »

Get Items

The items() method will return each item in a dictionary, as tuples in a list.

Example

Get a list of the key:value pairs

x = thisdict.items()

Try it Yourself »

The returned list is a view of the items of the dictionary, meaning that any changes done to the dictionary will be reflected in the items list.

Example

Make a change in the original dictionary, and see that the items list gets updated as well:

car = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = car.items()

print(x) #before the change

car["year"] = 2020

print(x) #after the change

Try it Yourself »

Example

Add a new item to the original dictionary, and see that the items list gets updated as well:

car = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

x = car.items()

print(x) #before the change

car["color"] = "red"

print(x) #after the change

Try it Yourself »

Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

Example

Check if "model" is present in the dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

if "model" in thisdict:

print("Yes, 'model' is one of the keys in the thisdict dictionary")

Change Values

You can change the value of a specific item by referring to its key name:

Example

Change the "year" to 2018:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict["year"] = 2018

Try it Yourself »

Update Dictionary

The update() method will update the dictionary with the items from the given argument.

The argument must be a dictionary, or an iterable object with key:value pairs.

Example

Update the "year" of the car by using the update() method:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict.update({"year": 2020})

Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

Example

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict["color"] = "red"

print(thisdict)

Try it Yourself »

Update Dictionary

The update() method will update the dictionary with the items from a given argument. If the item does not exist, the item will be added.

The argument must be a dictionary, or an iterable object with key:value pairs.

Example

Add a color item to the dictionary by using the update() method:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict.update({"color": "red"})

Removing Items

There are several methods to remove items from a dictionary:

Example

The pop() method removes the item with the specified key name:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict.pop("model")

print(thisdict)

Try it Yourself »

Example

The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict.popitem()

print(thisdict)

Try it Yourself »

Example

The del keyword removes the item with the specified key name:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

del thisdict["model"]

print(thisdict)

Try it Yourself »

Example

The del keyword can also delete the dictionary completely:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

del thisdict

print(thisdict) #this will cause an error because "thisdict" no longer exists.

Try it Yourself »

Example

The clear() method empties the dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

thisdict.clear()

print(thisdict)

Loop Through a Dictionary