Python Lists

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 are Tuple, Set, and Dictionary, all with different qualities and usage.

Lists are created using square brackets:

Example

Create a List:

list = ["app", "ban", "cher"]
print(list)

List Length

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

Example

Print the number of items in the list:

list = ["app", "ban", "cher"]
print(len(list))

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':

Example

What is the data type of a list?

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

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.

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])

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])

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:])

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)

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)

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)

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 specified index:

Example

Insert "watermelon" as the third item:

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

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)

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)

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)

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)

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 occurance:

Example

Remove the first occurance 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 index, 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 index:

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

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)

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])

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 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

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:

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

for x in fruits:
  if "a" in x:
    newlist.append(x)

print(newlist)

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

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

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

print(newlist)

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)

Example

Sort the list numerically:

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

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)

Another way to join two lists is 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)

List Methods

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