Python Lists:

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, changeable and indexed. 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.

List:

A list is a collection which is ordered and changeable. In Python lists are written with square brackets.

Example:

Create a List:

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

print(thislist)

Access Items:

You access the list items by referring to the index number:

Example:

Print the second item of the list:

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

print(thislist[1])

Negative Indexing:

Negative indexing means beginning 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 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 "orange":

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" and to the end:

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:

Example:

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

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

print(thislist[-4:-1])

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)

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)

You will learn more about for loops in our Python For Loops Chapter.

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")

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))

Add 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)

To add an item at the specified index, use the insert() method:

Example:

Insert an item as the second position:

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

thislist.insert(1, "orange")

print(thislist)

Remove Item:

There are several methods to remove items from a list:

Example:

The remove() method removes the specified item:

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

thislist.remove("banana")

print(thislist)

Example:

The pop() method removes the specified index, (or the last item if index is not specified):

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

thislist.pop()

print(thislist)

Example:

The del keyword removes the specified index:

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

del thislist[0]

print(thislist)

Example:

The del keyword can also delete the list completely:

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

del thislist

Example

The clear() method empties the list:

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

thislist.clear()

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 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)

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)

The list() Constructor

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

Example:

Using the list() constructor to make a List:

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

print(thislist)

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

Test Yourself With Exercises

Exercise:

Print the second item in the fruits list.

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

print(

)