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:

Example:
mylist = ["apple", "banana", "grapes"]
print(mylist)

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:

Example:
mylist = ["apple", "banana", "grapes", "apple", "grapes"] print(mylist)

List Length

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

Example:
print the number of items in the list
mylist = ["apple", "banana", "cherry","grapes"]
print(len(mylist))

List items data types

List items can be of any data type:
Example:
string, int and boolean data types
list1 = ["apple", "banana", "cherry","grapes"]
list2 = [1, 5, 7, 9, 3,11]
list3 = [True, False, False]

type()

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

Example:
mylist = ["apple", "banana", "cherry","grapes"]
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:
mylist = list(("apple", "banana", "cherry")) # note the double round-brackets print(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.

Access List Items

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

Example:
Print the third item of the list:
mylist = ["apple", "banana", "cherry","grapes"]
print(mylist[2])

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

Check if items exist

To determine if a specified item is present in a list use the in keyword:

Example:
mylist=["apple", "banana", "cherry","grapes"]
if "apple" in mylist:
print("Yes, 'apple is in the fruit list")

Change Item value

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

Example:
mylist = ["apple", "banana", "cherry","grapes"]
mylist[1] = "orange"
print(mylist)

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:
mylist = ["apple", "banana", "cherry", "orange", "kiwi", "mango","grapes"]
mylist[1:3] = ["orange", "watermelon"]
print(mylist)

Append items

To add an item to the end of the list, use the append() method:

Examples:
Using the append() method to append an item:
mylist = ["apple", "banana", "cherry","orange"]
mylist.append("grapes")
print(mylist)

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:
mylist = ["apple", "banana", "cherry","grapes"]
mylist.insert(1, "orange")
print(mylist)

Extend list

To append elements from another list to the current list, use the extend() method.
Example:
add the elements of mylist to mylist1
list1 = ["apple", "banana", "cherry"]
list2 = ["mango", "pineapple", "papaya"]
list1.extend(list2)
print(list1)

Remove list items (Remove specified Item)

Remove specified item
The remove() method removes the specified item.

Example:
mylist = ["apple", "banana", "cherry","orange"]
mylist.remove("banana")
print(mylist)

Remove list items (Remove specified Index)

The pop() method removes the specified index.

Example:
mylist = ["apple", "banana", "cherry","orange"]
mylist.pop(1)
print(mylist)

Clear the list

The clear() method empties the list.

The list still remains, but it has no content.

Example:
mylist = ["apple", "banana", "cherry","grapes"]
mylist.clear()
print(mylist)

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.
mylist = ["apple", "banana", "cherry","orange"]
for x in mylist:
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:
mylist = ["apple", "banana", "cherry","grapes"]
for i in range(len(mylist)):
print(mylist[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
mylist = ["apple", "banana", "cherry","orange"]
i = 0
while i < len(mylist):
print(mylist[i])
i = i + 1

Sort list alphanumerically

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

Example:
mylist = ["orange", "mango", "kiwi", "pineapple", "banana","grapes"]
mylist.sort()
print(mylist)

Sort Descending

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

Example:
mylist = ["orange", "mango", "kiwi", "pineapple", "banana","grapes"] mylist.sort(reverse = True)
print(mylist)

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.

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.

Use the copy() method

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

Use the list() method

mylist = ["apple", "banana", "cherry"]
mylist1 = list(mylist1)
print(mylist1)

Use the slice operator

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

Example:
mylist = ["apple", "banana", "cherry"]
mylist1 = mylist[:]
print(mylist1)

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:
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.

Method	Descriptions
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