Comprehension in Python

In this tutorial, you will learn about comprehension. Comprehension in Python provides a short and concise way to create a sequence (such as a list, dictionary, set, etc). Python support four types of comprehension which are a list, set, dictionary, and Generators Comprehension.

Before going through this Comprehension in Python article, please read the following articles.

• Python List
• Python set.
• Python dictionary.

What are Python Comprehensions?

Python comprehension provides a short and concise way to create a sequence like a list, dictionary, set and generator, etc. This is one of the most important concepts to create any sequence like a list, dictionary, set, or generator because it takes less time in comparison to traditional ways.

Python support 4 types of comprehension.

• List Comprehension
• Set Comprehension
• Dictionary Comprehension
• Generator Comprehension

Let’s understand all the Python comprehension one by one along with examples.

List Comprehension:

List comprehension provides a concise way to create a new list. list comprehension use square bracket ( [ ] ).

Syntax of List Comprehension

output_list = [expression for var in list]

Example: Creating a new one with another list using list comprehension

Here, I have an already defined list called old_list having some integer items and now I am about to create a new list using the list comprehension concept.

Suppose we want to create an output list that contains only the even number from 1 to 10 using another list. Then we have two ways to create an output list first is using Python for loop and the other is using list comprehension.

1. Create a new list without list comprehension

old_list = [1,2,3,4,5,6,7,8,9,10]
new_list = []

for var in old_list:
	if var % 2 == 0:
		new_list.append(var)
		
print("Output list without list comprehension: ",new_list)

Output

Output list without list comprehension:  [2, 4, 6, 8, 10]

2. Create a new list using list comprehension

old_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
new_list = [var for var in old_list if var % 2 == 0]
print("Output list using list comprehension: ", new_list)

Output

Output list using list comprehension:  [2, 4, 6, 8, 10]

Dictionary Comprehension

Dictionary comprehension looks like list comprehension. Dictionary comprehension provides a facility to create a new dictionary. Dictionary comprehension using an inside curly bracket ( { } ).

A dictionary comprehension takes the form {key: value for (key, value) in iterable}.

Syntax of Dictionary Comprehension:

output_variable = {key: value for (key, value) in iterable}

Example: Creating Dictionary using dictionary comprehension

Suppose we want to create a new dictionary that contains squares of numbers who exactly divisible by 3 between 1 to 20.

1. Create a new dictionary without dictionary comprehension.

old_list = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
new_dict = {}

for var in old_list:
	if var % 3 == 0:
		new_dict[var] = var * var

print("New dictionary without dictionary comprehension: ",new_dict)

Output

New dictionary without dictionary comprehension:  {3: 9, 6: 36, 9: 81, 12: 144, 15: 225, 18: 324}

2. Create a new dictionary without dictionary comprehension.

old_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
new_dict = {var: var * var for var in old_list if var % 3 == 0}
print("New Dictionary using dictionary comprehension:- ", new_dict)

Output

New Dictionary using dictionary comprehension:-  {3: 9, 6: 36, 9: 81, 12: 144, 15: 225, 18: 324}

you can check the type of the new dictionary using the type() function.

print(type(new_dict))

Set comprehension:

set comprehension is the same as a list comprehension. The only difference between list comprehension and set comprehension is, list comprehension uses a square bracket ( [ ] )but set comprehension uses a curly bracket ( { } ).

Syntax of set comprehension

output_variable = {expression for var in list}

Example: Creating a set using set comprehension

Here, In this example section. I have a students_name set having some student names included. Now, I am about to create a new set that will contain only student names starting with ‘Vi‘. I will create a new set using both ways, using set comprehension and without using set comprehension.

1. create a new set without set comprehension.

students_name = {"Vishvajit", "Vikas", "Vinod", "Akash", "Veronika", "Vaibhav", "John","Shashank"}
output_set = set()
for i in students_name:
    if i.startswith("Vi"):
        output_set.add(i)

print("Students whose name starts with 'Vi' :- ", output_set)

Output

Students whose name starts with 'Vi' :-  {'Vishvajit', 'Vikas', 'Vinod'}

You can check the data type of new_set using the type() function.

a = type(new_set)
print(a)

2. create a new set using set comprehension.

students_name = {"Vishvajit", "Vikas", "Vinod", "Akash", "Veronika", "Vaibhav", "John","Shashank"}
new_set = {i for i in students_name if i.startswith("Vi")}
print("Students whose name starts with 'Vi' :- ", new_set)

Output

Students whose name starts with 'Vi' :-  {'Vinod', 'Vishvajit', 'Vikas'}

Generator Comprehension:

Generator Comprehension is very similar to list comprehension. One difference between them is that generator comprehensions use circular brackets ( ( ) ) whereas list comprehensions use square brackets ( [ ] ). You have to keep one thing in mind, It always returns a generator object. To see the items of the generator object, you will have to convert them into the list, set, etc.

I have a student_names set having some names of students included, Now I am going to create a generator object that will contain only those student names which do not start with ‘Vi‘.

Example: Creating generator comprehension

students_name = {"Vishvajit", "Vikas", "Vinod", "Akash", "Veronika", "Vaibhav", "John","Shashank"}
generator_object = (i for i in students_name if not i.startswith("Vi"))
print("Generator Object :- ", generator_object)

Output

Generator Object :-  <generator object <genexpr> at 0x0000022A109099A0>

As you can see in the above output, how the generator object is looking where we are not able to see the items of the generator object.

Example: Converting generator object to the list

To convert any generator object to a list, use the list() function.

print(list(generator_object))
# Output:- ['Veronika', 'Shashank', 'Vaibhav', 'Akash', 'John']

Example: Converting generator object to set

The set() function will be used to convert the generator object to the set. As you can see below.

print(set(generator_object))
# Output:- {'Akash', 'Veronika', 'Shashank', 'John', 'Vaibhav'}

Note:- Order of items will be different, when you will convert generator object to set because set is unordered.

Example: Using for loop with generator object

Even, to access the items of the generator object, Python for loop can be used. Let see.

for i in generator_object:
    print(i)

Output

John
Shashank
Akash
Veronika
Vaibhav

Conclusion

In this article, you have seen approx all about Comprehension in Python. Python Comprehension is one the best way to create a list, set, Dictionary, and Generator comprehension in just one line. Python comprehension is a little faster than ways to create a list, set, and dictionary using the traditional way.

I will highly recommend you, Always use Python comprehension, when you have a requirement to create a list, set, or dictionary in just a few lines of code.

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