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In our previous Python article, we have explained a collection data type - tuple. Moving on in this tutorial, we are going to explain another collection data type provided by Python - list. Similar to a tuple, a list is also an ordered sequence of items. The items of a list are arranged in the order in which they are declared in the list, where the first item is stored at the index zero, the second item at the index one and so on.

Note : Each item within a list is an object.

• Important properties of a list



• A list is mutable i.e. existing items can be modified and we can append new items to it, whereas, a tuple is immutable.
• We can add duplicate items to a list.
• Each item in a list is accessible through its index.







• How to create a list in Python?


To create a list with items, we must declare its items within a pair of square brackets [ ], where each item is separated by the comma, operator. Let us show you a simple example of how to create a list of items.



#Python - Creating a list with items


random_info =['Keep Smiling', 19, 78.59, 'A']
print(random_info)




Output




['Keep Smiling', 19, 78.59, 'A']


In the example above, we have created a list named random_info, containing 4 items separated by the comma, operator and all the items are enclosed within a pair of square brackets [ ]. The items of the list are displayed in the order in which they were declared within a list.






• Accessing each item of a list


Items of a list are stored by using the zero-based indexing i.e. the first item of a list is stored at the index zero, the second element at the index one and so on.



#Python - Accessing each time of a list using its index.


misc =[19 ,-9, 9.99, 'Have a perfect day!']
print(misc)
print(misc[2])
print(misc[0])
print(misc[3])
print(misc[1])




Output




[19, -9, 9.99, 'Have a perfect day!']
9.99
19
Have a perfect day!
-9


In the example above, we have created a list named misc, containing 4 items separated by the comma, and enclosed within a pair of parenthesis [ ].
We have accessed each item of the list by accessing its index position :
• The third list element 9.99 is accessed using its index position 2.
• The first list element 19 is accessed using its index position 0.
• The fourth list element Have a perfect day! is accessed using its index position 3.
• The second list element -9 is accessed using its index position 1.






• Iterating over the items in a list using for loop


Using the membership operator in with the for loop, allows us to iterate over the items within a list. Let us see how we do this.



#Python - Iterating over the items in a list using for loop

# Creating a list
misc =[19 ,-9, 9.99, 'Have a perfect day!']

# Using for loop to iterate over items in a list
for val in misc:
        print(val)




Output




19
-9
9.99
Have a perfect day!








• A list is mutable


Once we have declared a list with its items, it is mutable i.e. its existing items can be modified and we can insert or append new items to it. Let us first see how we can modify a list once it is created.



#Python - Once declared, a list is mutable


misc2 =[2 ,-19, 17.5, 'Make your life productive!']

print('misc : ', misc2)

#Trying to insert a new item at index 1 in a list
misc2[1]= 'Hey'

#Trying to insert a new item at index 2 in a list
misc2[2]= 5

print('Modified misc : ', misc2)




Output




misc :  [2, -19, 17.5, 'Make your life productive!']
Modified misc :  [2, 'Hey', 5, 'Make your life productive!']


In the example above, we have created a list named misc2 with 4 items and printed its content. Next, in order to modify the list, we have inserted a new item at its index 1 and a new item at its index 2 and have printed the content of the modified list.

Note : Comparing to list, the modification of a tuple(once it is created with items) is not allowed, as it is immutable.






• Methods of list


Now let us see and discuss some important methods of list collection data type.



Methods	Description
index()	To find the index of an item in a list.
count()	Finds the number of times an item exists in a list.
insert()	Inserts an item at a specific index in a list.
append()	Adds an item or a list to the end of a list.
extend()	Extends a list to contain new items or another list.
copy()	Returns copy of a list.
clear()	Clears/removes all the items in a list.
pop()	Removes an item from a specific index in a list.
remove()	Removes a specific item in a list.
sort()	Sorts all the items in a list in an ascending order.
reverse()	Reverses the order of items in a list.






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• To find the index of an item in a list - index() method


To find the first index of an item within a list, we can use the index() method of the list data type. This method takes an item(whose index is to be searched) as an argument and returns its first index(if found) or else, it reports a ValueError, if the searched item does not exist within a list.



#Python - Using the index() method  of a list data type


misc =[-19, 2, 'think positive!', 2]
print('First index of item 2 : ', misc.index(2))




Output




First index of item 2 : 1


The above program finds the first index of item 2, which exists twice within the searched list but the function index returns only the first index of the searched item.






• To find the number of times an item exists in a list - count() method


We could store duplicate items in a tuple, but can we also store duplicate items in a list? The answer is, yes! And this raises a question - if we can find the number of times a specific item exists in a list? Let's see.

The list data type gives us another important method of list - count, using which we find the total number of times an item exists in a list. This method takes the value of an item(to be searched) as its argument and returns the total number of times it is contained in a list.



#Python - Using the count() method with a list


misc =[5 , -9, 'After a sad day comes a glad day!', 5]
print('misc : ',misc)
print('Total number of times item 5 is contained in a list : ', misc.count(5))




Output




misc :  [5, -9, 'After a sad day comes a glad day!', 5]
Total number of times item 5 is contained in a list :  2








• To find the total number of items in a list


We can use the len() function to find the total number of items in a list. This function takes a list as an argument and returns the total number of items it contains.



#Python - Using the len() function with a list


misc =[2019 , 9.999, 'Please, share your smile!']
print('misc : ', misc)
print('Total number of elements in list : ', len(misc))




Output




misc :  [2019, 9.999, 'Please, share your smile!']
Total number of elements in list :  2








• Concatenating list using arithmetic + operator


We could even create a new list by concatenating different lists, by using the arithmetic + operator. The arithmetic + operator is overloaded to add integers, floating values, concatenate string values and concatenate list.



#Python - Concatenating list using the arithmetic + operator


#First list
list1 =['A', -20]

#Second list
list2 =['have a good day', 2019]

#Printing lists
print('First List : ', list1)
print('Second List : ', list2)

#Creating a new list by concatenating existing lists
list3 = list1 + list2

#Printing new list
print('New List : ',list3)




Output




First List :  ['A', -20]
Second List :  ['have a good day', 2019]
New List :  ['A', -20, 'have a good day', 2019]


In the program just above, we have first created two list, and next we created a third list by concatenating the first two lists, by using the arithmetic + operator.






• Removing an item from a list using the remove() method


We could even remove the first occurence of an item from a list by calling the built-in remove() method. When calling the remove() method, we only need to pass the item that we want to remove from the list. Let us see how we use the remove() method.



#Python - Using the remove() method  of a list data type


misc =[-19, 2, 'think positive!', 2]
print('list : ', misc)

misc.remove(2)
print('updated list :',misc)




Output




list :  [-19, 2, 'think positive!', 2]
updated list : [-19, 'think positive!', 2]








• Removing an item from a list using the pop() method


We could even remove an item from a list by calling the built-in pop() method. When calling the pop() method, we only need to pass the index position(zero-based index) of the item that we want to remove from the list.

Note: The difference between the remove() and pop() method is that the remove() method removes the first occurence of an item that is passed to it, while the pop() removes an item by using the index(zero-based index) passed to it.

Let us see how we use the pop() method.



#Python - Using pop() method  of a list data type


misc =[-19, 2, 'think positive!', 2]
print('list : ', misc)

misc.pop(2)
print('updated list :',misc)




Output




list :  [-19, 2, 'think positive!', 2]
updated list : [-19, 2, 2]








• Appending a list using append() method


We could even add a new item to the end of a list by calling the append() method. This new item to be appended could just be a general object or a list object. The append() method is called by using a list object and it takes the object(to be appended) as an argument.



#Python - Extending a list to contain another list using extend() method


#First list
list1 =['A', -20]

#Second list
list2 =['have a good day', 2019]

#Printing lists
print('First List : ', list1)
print('Second List : ', list2)

#Calling append() method to append list2 to the end of list1 
list1.append(list2)

#Printing new list
print('Updating First List : ',list1)

#Calling append() method to append an object to the end of list1 
list1.append(8.5)

#Printing new list
print('Updating First List : ',list1)




Output




First List :  ['A', -20]
Second List :  ['have a good day', 2019]
Updating First List :  ['A', -20, ['have a good day', 2019]]
Updating First List :  ['A', -20, ['have a good day', 2019], 8.5]


First we have created two list objects and next we have appended the second list to the first list by using the append() method, which results in a modified first list, and finally we have appended a new value to the first list.

Note : Looking at the updated first list in the output, you should understand that by calling the append() method to append a list to another list, results a list within another list i.e. the second list is contained within the first list. But calling the append() method to append an object to a list will keep the original list as a simple list.






• Extending a list using extend() method

By using the extend() method we could extend a list to contain new items or another list. This new item should be a iterable object i.e. a list or a tuple. The extend() method is called by using a list object and it takes an iterable object as an argument.

#Python - Extending a list by calling the extend() method


#First list
list1 =['A', -20]

#Second list
list2 =['have a good day', 2019]

#Printing lists
print('First List : ', list1)
print('Second List : ', list2)

#Calling extend() method to extend list1 to the include a list2 i.e. iterable object
list1.extend(list2)

#Printing new list
print('Updating First List : ',list1)

#Calling extend() method to extend list1 to contain multiple items of a list i.e. iterable object.
list1.extend([29,  'haha'])

#Printing new list
print('Updating First List : ',list1)


#Calling extend() method to extend list1 to contain a tuple  i.e. iterable object.
list1.extend((2.5,  1978))

#Printing new list
print('Updating First List : ',list1)

Output

First List :  ['A', -20]
Second List :  ['have a good day', 2019]
Updating First List :  ['A', -20, 'have a good day', 2019]
Updating First List :  ['A', -20, 'have a good day', 2019, 29, 'haha']
Updating First List :  ['A', -20, 'have a good day', 2019, 29, 'haha', 2.5, 1978]

In the program just above, we have created two list objects and then we have extended the first list to contain the elements of the second list, which results in a modified first list.

Note : Looking at the updated first list in output, you should understand that by calling the extend() method leads to just one list i.e. the original list is extended to contain the items of iterable object(be it a list or a tuple).

Some important built-in functions to work with list

Functions	Description
max()	Returns the maximum value in a list.
min()	Returns the minimum value in a list.
len()	Returs the total length of a list.
sum()	Returns sum of all elements in a list.
sorted()	Returns a sorted copy of a list.
any()	Returns true if any element in a list is true, else false.
all()	Returns true if all elements in a list are true, else false.

#Python - Functions working with lists 


#First list
list1 =[19, -20, 209, 799.99, -15.4]

#Second list
list2 =[1, True, False, 0] # 1 is boolean true, 0  for boolean false
                          # True(with capital T) is boolean true, False(with capital F) is boolean false

#Printing lists
print('List1 : ', list1)
print('List2 : ', list2)



#Using max() function with a list.
print('Maximum value in List1 : ', max(list1))


#Using min() function with a list.
print('Minimum value in List1 : ', min(list1))


#Using sum() function with a list.
print('Sum of values in List1 : ', sum(list1))


#Using sorted() function with a list.
print('Sorted copy of List1 : ', sorted(list1))


#Using all() function with a list.
print('all() boolean true values in List2 : ', all(list2))


#Using any() function with a list.
print('any boolean true value in List2 : ', any(list2))

Output

List1 :  [19, -20, 209, 799.99, -15.4]
List2 :  [1, True, False, 0]
Maximum value in List1 :  799.99
Minimum value in List1 :  -20
Sum of values in List1 :  992.59
Sorted copy of List1 :  [-20, -15.4, 19, 209, 799.99]
all() boolean true values in List2 :  False
any boolean true value in List2 :  True

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