Python Lists

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

List

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 areTuple,Set, andDictionary, all with different qualities and usage.

Lists are created using square brackets:

Example

Create a List:

thislist = ["apple", "banana", "cherry"]
print(thislist)

List Items

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

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)

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))

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]

A list can contain different data types:

Example

A list with strings, integers and boolean values:

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

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))

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)

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, unchangeable*, and unindexed. No duplicate members.
Dictionary is a collection which is ordered** and changeable. No duplicate members.

Python - Access List Items

Access Items

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

Example

Print the second item of the list:

thislist = ["apple", "banana", "cherry"]
print(thislist[1])

Note: The first item has

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])

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])

Note: The search will start at ncluded and end at ot included.

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])

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")

Python - Change List Items

Change Item Value

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

Example

Change the second item:

thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)

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 ers 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)

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)

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)

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 specifie

Example

Insert "watermelon" as the third item:

thislist = ["apple", "banana", "cherry"]
thislist.insert(2, "watermelon")
print(thislist)

Note: As a result of the example above, the list will now contain 4 items.

Python - Add List Items

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)

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)

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)

Python - Remove List Items

Remove Specified Item

The remove() method removes the specified item.

Example

Remove "banana":

thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist)

If there are more than one item with the specified value, the remove() method removes the first occurrence:

Example

Remove the first occurrence of "banana":

thislist = ["apple", "banana", "cherry", "banana", "kiwi"]
thislist.remove("banana")
print(thislist)

Remove Specified Index

The pop() method removes the specified index.

Example

Remove the second item:

thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
print(thislist)

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

Example

Remove the last item:

thislist = ["apple", "banana", "cherry"]
thislist.pop()
print(thislist)

The del keyword also removes the specified values

Example

Remove the first item:

thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)

The del keyword can also delete the list completely

Example

Delete the entire list:

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

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)

Python - Loop Lists

Loop Through a List

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

Print all items in the list, one by one:

thislist = ["apple", "banana", "cherry"]
for x in thislist:
  print(x)

Loop Through the Index Numbers

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

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

Print all items by referring to their er:

thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
  print(thislist[i])

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 referring to their indexes.

Remember to increase the after each iteration.

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

thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
  print(thislist[i])
  i = i + 1

Looping Using List Comprehension

List Comprehension offers the shortest syntax for looping through lists:

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

thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]

Python - List Comprehension

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

The Syntax

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

Only accept items that are not "apple":

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

Condition

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

Iterable

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

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

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

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:

Set the values in the new list to upper case:

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

Return "orange" instead of "banana":

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

Python - Sort Lists

Sort List Alphanumerically

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

Sort the list alphabetically:

thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
print(thislist)

Sort Descending

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

thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
print(thislist)

Customize Sort Function

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

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)

Case Insensitive Sort

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

Perform a case-insensitive sort of the list:

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist)

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.

thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
print(thislist)

Python - Copy Lists

Copy a List

You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference tolist1, and changes made inlist1 will automatically also be made inlist2.

Example

Use the copy() method

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

thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)

Example

Use the list() method

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

thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)

Example

Use the slice Operator

You can also make a copy of a list by using the : (slice) operator:

thislist = ["apple", "banana", "cherry"]
mylist = thislist[:]
print(mylist)

Python - Join Lists

Join Two Lists

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

Example

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

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]

list3 = list1 + list2
print(list3)

Example

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

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]

for x in list2:
  list1.append(x)

print(list1)

Example

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

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]

list1.extend(list2)
print(list1)

Python - 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