Python Comprehensions
Table Of Contents:
- What Is Comprehension?
- List Comprehensions
- Dictionary Comprehensions
- Set Comprehensions
- Generator Comprehensions
(1) What Is Comprehension?
- Comprehension is the process of reducing the code length and being able to write it in a single statement.
- Using comprehension you can use filtering, mapping, and apply conditional logic to the members of a sequence (such as lists, set, dictionary etc.) in a single line.
- Comprehension allows us to filter the elements of the sequence and apply some conditional logic to an existing sequence, and it will return a new sequence.
Without Comprehensions:
numbers = [1,2,3,4,5,6,7,8,9,10]
even_numbers = []
for i in numbers:
if i % 2 == 0:
even_numbers.append(i)
print(even_numbers)Output:
[2, 4, 6, 8, 10]With Comprehensions:
numbers = [1,2,3,4,5,6,7,8,9,10]
even_numbers = [i for i in numbers if i % 2 == 0]
print(even_numbers)Output:
[2, 4, 6, 8, 10]Note:
- Here you can see that without comprehension it took 6 lines to complete the program.
- With comprehension, it took 3 lines to complete the code.
(2) List Comprehension?
- You can filter the elements of the list using list comprehensions in a single line.
- You can able to write the for loop and conditional statements inside square brackets [ ].
- The elements which pass the conditions will be stored inside the new list.
Syntax:
output_list = [var for var in input_list if (var satisfies this condition)]Note:
- The first part ‘var’ is the filtered-out value, that is going to be stored inside the new list.
- The second part is the for loop, which is going to iterate over the existing list.
- The third part is the conditional statement, which will check if the condition is satisfying or not.
Example-1:
vowels = ['a','e','i','o','u']
input_string = 'Abhispa'
vowels_list = [v for v in input_string if v.lower() in vowels]
print(vowels_list)Output:
['A', 'i', 'a']Note:
- In this program, we have filtered out all the vowels from the input string.
Example-2:
name = 'Abhispa Pattnaik'
letters = [i for i in name]
print(letters)Output:
['A', 'b', 'h', 'i', 's', 'p', 'a', ' ', 'P', 'a', 't', 't', 'n', 'a', 'i', 'k']Note:
- In this program, we are iterating over the name and storing it inside the list.
(3) Dictionary Comprehension?
- Dictionary Comprehension works the same as List Comprehension.
- it uses curly braces { } instead of square brackets [ ] .
- You can filter out the dictionary elements by using this technique.
Syntax:
output_dict = {key:value for (key, value) in iterable if (key, value satisfy this condition)}Example-1:
numbers = [2,3,4,5,6]
squares = {key:key**2 for key in numbers}
print(squares)Output:
{2: 4, 3: 9, 4: 16, 5: 25, 6: 36}Note:
- In this program, we are creating a dictionary, which will contain squares of the numbers from the list.
Example-2:
names = ['Subrat','Abhispa','Arpita','Sonali','Ruchi']
marks = [59,69,79,89,99]
names_marks = {names:marks for names,marks in zip(names,marks)}
print(names_marks)Output:
{'Subrat': 59, 'Abhispa': 69, 'Arpita': 79, 'Sonali': 89, 'Ruchi': 99}Note:
- In this program, we are creating a name and mark dictionary.
(4) Set Comprehension?
- Set Comprehension works the same as Dictionary Comprehension.
- it uses curly braces { } instead of square brackets [ ] .
- You can filter out the set elements by using this technique.
Syntax:
output_list = {var for var in input_list if (var satisfies this condition)}Example-1:
numbers = [1,2,1,3,4,3,5,6,5]
unique = {i for i in numbers}
print(unique)Output:
{1, 2, 3, 4, 5, 6}Note:
- In this program, we will create a set from the list.
- The set will contain unique elements only.
Example-2:
numbers = [2,2,4,4,5,6,8,8,9,10]
even_numbers = {i for i in numbers if i % 2 == 0}
print(even_numbers)Output:
{1, 2, 3, 4, 5, 6}Note:
- In this program, the set will contain a unique even number of elements from the list.
(5) Generator Comprehension?
- Generator Comprehensions are very similar to list comprehension.
- One difference between them is that generator comprehensions use circular brackets ( ) whereas list comprehensions use square brackets [ ].
- The major difference between them is that generators don’t allocate memory for the whole list.
- Instead, they generate each value one by one which is why they are memory efficient.
Syntax:
output_list = (var for var in input_list if (var satisfies this condition))Example-1:
numbers = [2,2,4,4,5,6,8,8,9,10]
even_numbers = (i for i in numbers if i % 2 == 0)
for var in even_numbers:
print(var, end = ' ')Output:
2 2 4 4 6 8 8 10 Note:
- Here even_numbers is a generator object, which we need to iterate to get the elements.
Example-2:
name = 'Subrat'
letters = (i for i in name)
for i in letters:
print(i)Output:
S
u
b
r
a
tNote:
- Here we are iterating a name and storing it inside a generator object.
- We need to iterate over the generator object to get the individual letters.