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:

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

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.

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:

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

List Length

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

List Items - Data Types

List items can be of any data type:

list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]

A list can contain different data types:

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

type()

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

mylist = ["apple", "banana", "cherry"]
print(type(mylist))
#<class 'list'>

The list() Constructor

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

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

Access Items

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

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

Negative Indexing

Negative indexing means start from the end

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

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.

Return the third, fourth, and fifth item:

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:5])

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

</aside>

Range of Negative Indexes

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

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:

Check if "apple" is present in the list:

thislist = ["apple", "banana", "cherry"]
if "apple" in thislist:
   print("Yes, 'apple' is in the fruits list")

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 index number.

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

Print all items by referring to their index number:

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

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

List Methods

The list data type has some more methods. Here are all of the methods of list objects:

list.append(x) Add an item to the end of the list. Equivalent to a[len(a):] = [x].

list.extend(iterable) Extend the list by appending all the items from the iterable. Equivalent to a[len(a):] = iterable.

list.insert(i, x) Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a), x) is equivalent to a.append(x).

list.remove(x) Remove the first item from the list whose value is equal to x. It raises a ValueError if there is no such item.

list.pop([i]) Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list. (The square brackets around the i in the method signature denote that the parameter is optional, not that you should type square brackets at that position. You will see this notation frequently in the Python Library Reference.)

list.clear() Remove all items from the list. Equivalent to del a[:].

list.index(x[, start[, end]]) Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item. The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

list.count(x) Return the number of times x appears in the list.

list.sort(***, key=None, reverse=False) Sort the items of the list in place (the arguments can be used for sort customization, see sorted() for their explanation).

list.reverse() Reverse the elements of the list in place.

list.copy() Return a shallow copy of the list. Equivalent to a[:].