Python - Lists

 

Hello Technotizers, in this article we will try to give you detailed information on Python – Lists and their methods. Lists are using to store many items in a single variable. They are among the 4 in-built data types of Python that are used to store data, the other 3 being Tuples, Sets and Dictionary.

Lists are created using square brackets:

city = ['Mumbai','Delhi','Chennai','Kolkata']
print(city)                                  

The output is obtained as:

['Mumbai', 'Delhi', 'Chennai', 'Kolkata']    

Some points to be remembered:

·         List items are ordered, changeable and allow duplicate values.

·         List items are indexed, the first item is indexed [0] and so on.

·         A list item can be of any data type like int, string or Boolean. A list can also contain items of mixed data types.

 

Examples:

All the below given lists are valid in Python.

list2 = [1, 4, 24, 14, 8]            
list3 = [True, False, False]         
list4 = ["abc", 34, True, 40, "male"]

 

·         The list() constructor:

It is another way of creating a list.

city = list(('Mumbai','Delhi','Chennai','Kolkata'))

So now let’s move forward!!  

• Accessing list elements:

As seen in strings, list items are also accessed in the same way as in strings. List items are indexed so they can be accessed with the help of indexes. The indexing starts from 0 as usual. Negative indexing means that the indexing begins from the end. -1 is the last index, -2 the second last and so on. A range of indexes can also be specified form where to start (inclusive) and where to end (exclusive).

city = ['Mumbai','Delhi','Chennai','Kolkata','Surat','Indore']
#referring index                                              
print(city[2])                                                
#negative indexing                                            
print(city[-1])                                              
#range of indexes                                             
print(city[1:4])                                              
#range of negative indexes                                    
print(city[-3:-1])                                            

The output is obtained as:

Chennai                        
Indore                         
['Delhi', 'Chennai', 'Kolkata']
['Kolkata', 'Surat']           

 

• Changing list elements:

Index number is referred to change the value of an item. To change a range of specific items, a new list of values is defined, and is referred to the range of indexes where we want to put the new values.

#changing an item                            
city = ['Mumbai','Delhi','Chennai','Kolkata']
city[1] = 'Surat'                            
print(city)                                  
#changing range of items                     
city = ['Mumbai','Delhi','Chennai','Kolkata']
city[1:3] = ['Surat','Indore']               
print(city)                                  

The output is obtained as:

['Mumbai', 'Surat', 'Chennai', 'Kolkata']
['Mumbai', 'Surat', 'Indore', 'Kolkata'] 

If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly.

 

• Adding list elements:

To add an item at the end of the list, append() method is used.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.append('Surat')                         
print(city)                                  
#to store the item permanently in the list   
city = city.append('Surat')                  

The output is obtained as:

['Mumbai', 'Delhi', 'Chennai', 'Kolkata', 'Surat']

To insert an item at a specific index, insert() method is used.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.insert(1,'Surat')                       
print(city)                                  

The output is obtained as:

['Mumbai', 'Surat', 'Delhi', 'Chennai', 'Kolkata']

 

• Extend list:

The extend() method adds items from another list to the existing list.

city1 = ['Mumbai','Delhi','Chennai','Kolkata']
city2 = ['Surat','Indore','Nagpur']           
city1.extend(city2)                           
print(city1)                                  

The output is obtained as:

['Mumbai', 'Delhi', 'Chennai', 'Kolkata', 'Surat', 'Indore', 'Nagpur']

The extend() method does not have to append only lists, you can add any iterable object (tuples, sets, dictionaries etc.).

 

• Remove list items:

The remove() method removes a specific item.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.remove('Delhi')                         
print(city)                                  

The output is obtained as:

['Mumbai','Chennai', 'Kolkata']

The pop() method removes a specified index. If the index is not specified then the last item is removed.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.pop(1)                                  
print(city)                                  
#without specifying index                    
city = ['Mumbai','Delhi','Chennai','Kolkata']
city.pop()                                   
print(city)                                  

The output is obtained as:

['Mumbai', 'Chennai', 'Kolkata']
['Mumbai', 'Delhi', 'Chennai']  

The ‘del’ keyword is also used to remove specified index. The ‘del’ keyword can also entirely delete the list.

city = ['Mumbai','Delhi','Chennai','Kolkata']
del city[1]                                  
print(city)                                  
#delete entire list                          
del city                                     

The output is obtained as:

['Mumbai', 'Chennai', 'Kolkata']

The clear() method clears the list. The list still exists but without any content.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.clear()                                 
print(city)                                  

The output is obtained as:

[]

The output shows that the list is now empty.

 

• Looping a list:

A for loop is used to loop through a list. 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.

city = ['Mumbai','Delhi','Chennai','Kolkata']
for i in city:                               
    print(i,end=" ")                         
print('\n')                                  
for i in range(len(city)):                   
    print(city[i],end=" ")                   

The output is obtained as:

Mumbai Delhi Chennai Kolkata
Mumbai Delhi Chennai Kolkata

The len() method is used to get the length of a list.

To know more about for loops, refer the article Python - Loops

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.

city = ['Mumbai','Delhi','Chennai','Kolkata']
i = 0                                        
while i<len(city):                           
    print(city[i],end=" ")                   
    i = i+1                                  

The output is obtained as:

Mumbai Delhi Chennai Kolkata

 

• Sort Lists:

The sort() method is used to sort the list in ascending order by default. The lists that contain strings as items are sorted in alphabetical order.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.sort()                                  
print(city)                                  
numbers = [12,98,5,7,25]                     
numbers.sort()                               
print(numbers)                               

The output is obtained as:

['Chennai', 'Delhi', 'Kolkata', 'Mumbai']
[5, 7, 12, 25, 98]                       

To sort the list in descending order, use the keyword argument reverse=True.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.sort(reverse=True)                      
print(city)                                  
numbers = [12,98,5,7,25]                     
numbers.sort(reverse=True)                   
print(numbers)                               

The output is obtained as:

['Mumbai', 'Kolkata', 'Delhi', 'Chennai']
[98, 25, 12, 7, 5]                       

You can also customize your own function by using the keyword argument key = function. The function will return a number that will be used to sort the list (the lowest number first).

def my_func(n):            
    return abs(n-50)       
numbers = [100,65,50,45,80]
numbers.sort(key=my_func)  
print(numbers)             

The output is obtained as:

[50, 45, 65, 80, 100]

The above code sorts the numbers based on how much close it is to number 50.

Note: By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters.

 

To obtain the reverse of a list without sorting, the reverse() method is used.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city.reverse()                               
print(city)                                  

The output is obtained as:

['Kolkata', 'Chennai', 'Delhi', 'Mumbai']

 \

• Copy Lists:

The copy() method is used to copy one list into another. Also the list() constructor is used to do so.

city = ['Mumbai','Delhi','Chennai','Kolkata']
city2 = city.copy()                          
print(city2)                                 
numbers = [12,98,5,7,25]                     
numbers2 = numbers.copy()                    
print(numbers2)                              

The output is obtained as:

['Mumbai', 'Delhi', 'Chennai', 'Kolkata']
[12, 98, 5, 7, 25]                       

 

• Join Lists:

One of the easiest way to join lists is using the + operator. Another way to join two lists is by appending all the items from list2 into list1, one by one. Or you can use the extend() method, whose purpose is to add elements from one list to another list.

city = ['Mumbai','Delhi','Chennai','Kolkata']
numbers = [12,98,5,7,25]                     
#using + operator                            
print(city+numbers)                          
#using append() method                       
for i in numbers:                            
    city.append(i)                           
print(city)                                  
#using extend() method                       
city = ['Mumbai','Delhi','Chennai','Kolkata']
numbers = [12,98,5,7,25]                     
city.extend(numbers)                         
print(city)                                  

The output is obtained as:

['Mumbai', 'Delhi', 'Chennai', 'Kolkata', 12, 98, 5, 7, 25]
['Mumbai', 'Delhi', 'Chennai', 'Kolkata', 12, 98, 5, 7, 25]
['Mumbai', 'Delhi', 'Chennai', 'Kolkata', 12, 98, 5, 7, 25]
 

• List comprehension:

List comprehension offers a shorter syntax when we need to create a mew list based on an existing list.

Let’s see an example:

Consider we want to create a list that has names of cities that contain the letter ‘a’ in them, based on an existing list. For this we write the code as:

city = ['Mumbai','Delhi','Chennai','Kolkata']
new = []                                     
for i in city:                               
    if "a" in i:                             
        new.append(i)                        
print(new)                                   

But with list comprehension, all of this code can be done in one line.

city = ['Mumbai','Delhi','Chennai','Kolkata']
new = [i for i in city if "a" in i]          
print(new)                                   

Both the codes give the same outputs as below.

['Mumbai','Chennai','Kolkata']

The syntax for list comprehension:

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

Some other examples:

city = ['Mumbai','Delhi','Chennai','Kolkata']       
new = ['hello' for i in city]                       
print(new)                                          
new = [i if i != "Mumbai" else "Surat" for i in city]
print(new)                                          
new = [i for i in range(10) if i < 5]               
print(new)                                          

The output is obtained as follows:

['hello', 'hello', 'hello', 'hello']    
['Surat', 'Delhi', 'Chennai', 'Kolkata']
[0, 1, 2, 3, 4]                         

 

• List methods:

Python offers a bunch of list methods given below:

Method	Purpose
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

 

With this we come to an end of this article. Hope it was helpful. Do provide your feedback and ideas through comments, it would be highly appreciated. See you soon!

Keep coding and exploring new techs!!