Python Datatype - Part 2 (Lists)

Python Lists

There are datatype use to store multiple items in single variable. Python have 4 built-in datatype that stores collections of data, they are Tuples, List, Set and Dictionary.

Lists are created using square brackets

mylist = ["Red", "Orange", "Green", "Black"]
print(mylist)        

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.

List items can be of any data type

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

Ordered: When we say that lists are ordered, it means that the items have a defined order. 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

mylist = ["Red", "Orange", "Green", "Black", "Red", "Orange",]
print(mylist)        

List len()

len() method is use to determine the number of items in a list

mylist = ["Red", "Orange", "Green", "Black"]
print(len(mylist))        

A list can contain different data types

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

Access List Items

To access the list items, you will need to use the index number to reference them.

Note: The first item has index 0.

mylist = ["Red", "Orange", "Green", "Black"]
print(mylist[1])        

Negative Indexing

Negative indexing means starting the index check from the end. -1 refers to the last item, -2 refers to the second last item etc.

mylist = ["Red", "Orange", "Green", "Black"]
print(mylist[-1])        

Range of Indexes

You can specify a range of indexes by specifying where to start and where to end the range.

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).

To start at the first item leave out the start value:

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

To go to the end item leave out the end value

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[4:])        

Range of Negative Indexes

Negative index is use to start the search from the end of the list

mylist = ["Red", "Orange", "Green", "Black", "Blue", "Yellow", "Brown", "White"]
print(mylist[-4:-1])        

Check if Item Exists

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

mylist = ["Red", "Orange", "Green", "Black", "Blue", "Yellow", "Brown", "White"]

if "Blue" in mylist:
   print("Yes, 'Blue' is in the color list")
else:
   print("Yes, 'Blue' is not in the color list")        

Change List Items

To change the value of a list item, reference the index number

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist[1] = "Blue"
print(mylist)        

Change a Range of Item Values

To change value of item in a range, define a new list of the value and reference to the range of index

mylist = ["Red", "Orange", "Green", "Black", "Blue", "Yellow", "Brown", "White"]
print(mylist)

mylist[2:5] = ["LightGreen", "Ash", "LightBlue"]
print(mylist)        

Add List Items

append() method is use to add new item to the end of the list

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist.append("Blue")
print(mylist)        

Insert Items

insert() method is use to add new item at a specified index

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist.insert(0, "Blue")
print(mylist)        

Extend List

To add items of a list to another list, the extend() method is used

mylist = ["Red", "Orange", "Green"]
mylist2 = ["Black", "Brown", "Blue"]

print(mylist)

mylist.extend(mylist2)
print(mylist)        

Extend method can also be use to add other datatype that stores collections of data, like Tuples, Set and Dictionary to a List.

mylist = ["Red", "Orange", "Green"]
mytuple = ("Black", "Brown", "Blue")

print(mylist)

mylist.extend(mytuple)
print(mylist)        

Remove Specific Item

remove() method can be used to remove specific item from the list

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist.remove("Orange")
print(mylist)        

Note: If there are more than one item with the specified value, the remove() method removes the first occurrence

Remove Specific Index

To remove item at a specific index, the pop() method is used

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist.pop(0)
print(mylist)        

The del keyword can also remove item at specific index from a list

mylist = ["Red", "Orange", "Green"]
print(mylist)

del mylist[2]
print(mylist)        

Clear the List

To clear all items in a list the clear() method can be applied

mylist = ["Red", "Orange", "Green"]
print(mylist)

mylist.clear()
print(mylist)        

Loop Lists

You can loop through a list to access the items or perform actions on/using the items. There are various option available to do that:

- for loop:

mylist = ["Red", "Orange", "Green", "Blue"]

for x in mylist:
  print(x)        

- Loop Through the Index Numbers: Use the range() and len() functions to create a suitable iterable.

mylist = ["Red", "Orange", "Green", "Blue"]

for i in range(len(mylist)):
  print(mylist[i])        

- Using a While Loop: Use the len() function to determine the length of the list

mylist = ["Red", "Orange", "Green", "Blue"]

i = 0
while i < len(mylist):
  print(mylist[i])
  i = i + 1        

- Looping Using List Comprehension:

mylist = ["Red", "Orange", "Green", "Blue"]

[print(x) for x in mylist]        

List Comprehension

Syntax:

newlist = [expression for item in iterable if condition == True]        

To create a new list based on application of some condition to an existing list, comprehension is the easiest way. Without comprehension you will have to write a loop statement with conditional test like below:

mylist = ["Red", "Orange", "Green", "Blue"]
newlist = []

for x in mylist:
  if "r" in x:
    newlist.append(x)

print(newlist)        

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

mylist = ["Red", "Orange", "Green", "Blue"]
newlist = [x for x in mylist if "r" in x]

print(newlist)        

Sort Lists

Sort List Alphanumerically: sort() method is use which sort in ascending order by default

mylist = ["Red", "Orange", "Green", "Blue"]

mylist.sort()
print(mylist)        

Sort Descending

mylist = ["Red", "Orange", "Green", "Blue"]

mylist.sort(reverse = True)
print(mylist)        

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)        

Perform a case-insensitive sort of the list by using str.lower as a key function

thislist = ["banana", "Orange", "Kiwi", "cherry"]

thislist.sort(key = str.lower)
print(thislist)        

Reverse Order

reverse() is use to reverse the order of a list

thislist = ["banana", "Orange", "Kiwi", "cherry"]

thislist.reverse()
print(thislist)