Python 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:

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

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"]
print(thislist)

List Length

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

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

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))

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)

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.

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.

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.

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

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

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:

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

Range of Negative Indexes

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

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:

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:

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:

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:

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:

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:

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:

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:

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.

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.).

thislist = ["apple", "banana", "cherry"]
thistuple = ("kiwi", "orange")
thislist.extend(thistuple)
print(thislist)

Remove Specified Item

The remove() method removes the specified item.

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:

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

Remove Specified Index

The pop() method removes the specified index.

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

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

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

The del keyword also removes the specified index:

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

The del keyword can also delete the list completely.

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

Clear the List

The clear() method empties the list.

The list still remains, but it has no 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:

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.

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

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 referring to their indexes.

Remember to increase the index by 1 after each iteration.

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:

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

Sort List Alphanumerically

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

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

Sort Descending

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

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

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):

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:

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

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:

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)

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().

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

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

list3 = list1 + list2
print(list3)

List Methods

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

Below YouTube content is helpful for better understanding: