Python List

By | May 13, 2019

In this tutorial, we will discuss one of the most important python datatypes. We will learn how to create, slice, add and remove a list.

Table of Content:

  • How to create a list
  • How to access the elements from a list
  • How to slice list in python
  • How to add elements in a list
  • How to remove an element from a list
  • List methods
  • List operation in python

What is a list and how to create a list in python?

A list is a mutable collection of different data types and to create a list we place all those different elements inside a square bracket [] and separate them with a comma (,).

Let’s create a list:

list_1 = [1,2,3,4,5,6,7]
list_2 = [1,"hello", "world", None, True]

we can even place a list inside a list

list_3 = [1,2,[3,4],"Hello"]

Access elements from a list.

To access an element or value stored in a list can be accessed through the indexing. Though when we initialize values inside the brackets [], and create the list all those values stored in contiguous memory location s and an index number is associated with every value or element inside a list.

So indexing is a method of retrieving values from a list using the integers values. By conventional indexing start with 0 and end with a number less than total numbers of elements inside a list. It means that 0 index is associated with the first element of the list.

If we try to access an element of list which does not have a corresponding index number then it will throw an Index error.

Syntax of accessing an element from a list

List_name [ index_number]

Let’s understand it with an example.

list_1 = [1,2,3,4,[6,7,8],"hello"]

print(list_1[0])
print(list_1[1])
print(list_1[2])
print(list_1[3])
print(list_1[4])
print(list_1[5])

#Output

1
2
3
4
[6, 7, 8]
Hello

Negative Indexing:

In python we can use a negative integer as indexing here -1 refers to the last element of the list and so on.

Let’s understand it with an example:

list_1 = [1,2,"hello",None]

print(list_1[-1])
print(list_1[-3])

#Output

None
2

Python List Slicing:

In python, you can control the sequence of elements and can access it. In list slicing, we can access a range of elements using the indexing and colon.

Syntax of list slicing:

list_name[starting_index : ending_index]

let’s understand it with an example:

list_1 = ["a","b","c","d","e","f","g"]

print(list_1 [2 : 5])
print(list_1[:])

#Output
['c', 'd', 'e']
['a', 'b', 'c', 'd', 'e', 'f', 'g']

Change or add elements to a list:

As we have defined that a list is a mutable collection by mutable mean that the values of the list can be altered with the assignment operator (=).

And we can also add a new element in a predefined list using a method (a function) known as append(). append() add a new element at the last of the position of the list.

Syntax to perform append() method on a list

list_name.append(new_item)

let’s understand it with an example:

my_list =["a",1,"b",2,"d",3]

my_list[4] = "c" #Here value d will change with c
my_list.append("d")
my_list.append(4)

print(my_list)

#Output

['a', 1, 'b', 2, 'c', 3, 'd', 4]

+ and * operator on a list:

We use the + operator to concatenate 2 list and * operator to create a multiple copies of items present in the list.

Syntax of using + and * operator on a list:

list_1 + list_2

list_1 * any_natural_number

Let’s understand it with an example:

list_1 =[1,2,3,4]
list_2 =["a","b","c","d"]

list_3 = list_1 + list_2

print(list_3)
print(list_3 *3 )

#Output

[1, 2, 3, 4, 'a', 'b', 'c', 'd']

[1, 2, 3, 4, 'a', 'b', 'c', 'd', 1, 2, 3, 4, 'a', 'b', 'c', 'd', 1, 2, 3, 4, 'a', 'b', 'c', 'd']

To delete elements from a list

There is keyword known as del which we can use to delete the entire list or a specific element.

Let’s understand it with an example

list_2 =["a","b","c","d"]

del(list_2[2])
del (list_2[2])

print(list_2)

#Output

['a', 'b']

Apart from del keyword, there are many other methods associated with the list that helps in delete elements from a list such as pop() and remove().

List Methods:

In python, the list has some methods and these methods are:

append() - Add an element to the end of the list

extend() - Add all elements of a list to the another list

insert() - Insert an item at the defined index

remove() - Removes an item from the list

pop() - Removes and returns an element at the given index

clear() - Removes all items from the list

index() - Returns the index of the first matched item

count() - Returns the count of the number of items passed as an argument

sort() - Sort items in a list in ascending order

reverse() - Reverse the order of items in the list

copy() - Returns a shallow copy of the list. 

List Comprehension:

A list comprehension a shorthand technique of creating a list using a for loop statement

Syntax of creating a list using list comprehension

list_name = [items for statement on items]

let’s understand it with an example

list_1 = [x**2 for x in range(10)]

print(list_1)

#Output

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

In keyword with a list:

If we want to know whether an item is presented in the list or not to do so we use the in keyword with the list. If the item is present in the list, we get a Boolean value True if not we get False.

Let’s understand it with an example

my_list = [1,2,"a","b"]

print( "a" in my_list)
print( "c" in my_list)

#Output

True
False

List as an Iterable:

As we know that list is iterable, like string so we can also use a list with the for-loop statement so the for loop can iterate through it.

For example:

for i in [1,2,3,4,5]:
    print(i)

#Output

1
2
3
4
5

Python Tutorials


Introduction

Flow Control

Functions

Datatypes

File Handling

Object & Class

Additional Tutorials

 

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