The Python List Data Structure

Introduction

Data structures are a very important in any programming language and python is no exception. They are of great importance when stroring and organizing data. In this article, we will have a top overview one of the most commonly used data structure in python, the list. We will be looking at what a list is, it's use cases, inbuilt methods and inbuilt operations. Ok let's dive in.

What is a list in python?

A list is a series of one or more items seperated by commas enclosed in square brackets.

my_list = [1,2,3,True,'string_elem',{3},(3,4),{'f':55}] # could also be created with the list(function)

It is a mutable data structure which means that it can be changed after it has been created unlike the python tuple.

my_list[1] = 'new_item'
print(my_list) # output -> [1,'new_item',3,True,'string_elem',{3},(3,4),{'f':55}]

It is also an ordered data structure which means the list retains the order of the items specified at the time of list creation unless explicitly modified.

my_list = [x for x in range(1,10)] # list comprehension to create a list of numbers 1 through 9 in that order
print(my_list) # output [1,2,3,4,5,6,7,8,9]

The items in a list can be of any data type. The items in a list are accessed using their index. A list index is the position of an item in a list. The first item in a list has an index of 0, the second 2 and so on. One could also use -1 in reference to the index of the last item , or -2 for the second last and so on. The index of an item in a list can be accessed using the index() method. my_list.index(3) # returns 2 A list can not be used as a key in a dictionary i.e. it is unhashable. Trying to do this would throw an error.

methods and operations supported by a list

adding or removing items from a list
sorting a list
reversing a list
finding the length of a list
finding the index of an item in a list
finding the count of an item in a list
finding the maximum and minimum item in a list
finding the sum of items in a list etc etc

methods explained

my_list = ['x',3,5,True,('d')] # sample list to use for example code
adding or removing items from a list
- append() - adds an item to the end of a list
```
my_list.append('new_elem')
print(my_list[-1]) # prints out  'new_elem'
```
- extend() - adds multiple items to the end of a list
```
new_list = ['f','y','z']
my_list.extend(new_list)
print(my_list) # prints out ['x',3,5,True,'new_elem','f','y','z']
```
- insert() - adds an item at a specified index
```
my_list.insert(3,'replaced') # replaces the item at index 3 with the string 'replaced'
```
- remove() - removes the first occurence of an item from a list
```
my_list.remove('x') # removes first occurence of x from the list
```
- pop() - removes an item from a list at a specified index and returns the item. If no index is specified, the last item is removed and returned.
- clear() - removes all items from a list
- del - removes an item from a list at a specified index
sorting a list
- sort() - sorts a list in ascending order and does this in place.
```
my_list.sort() #would sort my_list in ascending order , return None
```
- sorted() - sorts a list in ascending order and returns a new list , the original list is left intact. To sort in descending order , set the key word argument reverse to True.
```
sorted_list = sorted_list(my_list,reverse=True) # returns a new list sorted in descending order and stores it in sorted_list
```
reversing a list in place
- reverse my_list.reverse() returns None
finding the length of a list
- len len(my_list) returns an integer denoting length of the list
finding the count of an item in a list
- count my_list.count('elem_whose_freq_is_needed') returns how often the element passed on as argument appears in the list.
finding the maximum and minimum item in a list
- max max(my_list) returns integer denoting largest list element
- min min(my_list) returns integer denoting smallest or lowest value of list
finding the sum of items in a list
- sum sum(my_list) like the name suggests returns a sum of the elements in the list.
- The methods max, min, sum work on lists filled with only numerical values. We could find more of the methods supported by the list using the python inbuilt function , dir i.e. dir(list_name).

Use cases

storing a list of items with or without duplicates
storing a list of items in a specific order
storing a list of items with or without a specific data type

Wrapping up

In this article we have looked at what a list is, some of its use cases,the inbuilt methods and operations. We also looked at how to use the list methods and operations. This is only a small portion of what lists can be used for , more in depth information could be obtained from the official python documentation. I hope you found this article helpful. If you have any questions or comments, please leave them in the comment section below. Thank you for reading :).

kithstack's blog