Python List Comprehension

Way to create a new list from iterable by transforming the items.

Syntax:

[ output_expression for item in iterable [ conditions ] ]

numbers = [1, 2, 3, 4, 5, 6]
squares = [number**2 for number in numbers]
print(squares)
#Output: [1, 4, 9, 16, 25, 36]

Using if condition

We can use if condition to filter out some elements.

numbers = [1, 2, 3, 4, 5, 6]
squares = [number**2 for number in numbers if number > 2]
print(squares)
#Output: [9, 16, 25, 36]

Using multiple if condition (ANDing)

We can use multiple if conditions to filter out elements. All if conditions will be ANDed

numbers = [1, 2, 3, 4, 5, 18, 30]
list_1 = [number for number in numbers
          if number % 2 == 0
          if number % 3 == 0]
print(list_1)
#Output: [18, 30]

Using ‘in’ statement (ORing)

'in' keyword can be used to create a condition statement.

numbers = [1, 2, 3, 4, 5, 18, 30]
list_2 = [number for number in numbers
          if number % 3 in (1, 2)]
print(list_2)
#Output: [1, 2, 4, 5]

Using if condition in output expression

If condition can be used to alter the resulting list.

numbers = [1, 2, 3, 4, 5]
list_3 = ["Even" if number % 2 == 0
          else "Odd" for number in numbers]
print(list_3)
#Output: ['Odd', 'Even', 'Odd', 'Even', 'Odd']

Using the if-else statement

Ladder of if-else statement can be used to alter the resulting list.

numbers = [25, 35, 50, 90]
list_4 = ["First" if number >= 60
          else "Second" if 45 <= number < 60
          else "Third" if 33 <= number < 45
          else "Failed"
          for number in numbers]
print(list_4)
# Output: ['Failed', 'Third', 'Second', 'First']

Nesting list comprehension

Nesting of list comprehension is just like the nested for loops.

list_6 = [[n for n in range(1, 4)] for number in range(3)]
print(list_6)
# Output: [[1, 2, 3], [1, 2, 3], [1, 2, 3]]

Dictionary comprehension

Dictionary comprehension works the same as lists. The difference is that we will work on (key, value) pairs.

dict_1 = {'a': 1, 'b': 3, 'c': 4}
dict_2 = {key: value**2 for key, value in dict_1.items()}
print(dict_2)
# Output: {'a': 2, 'b': 6, 'c': 8}

Set comprehension

Set can be created just like the list is comprehended.

numbers = [1, 2, 3, 4, 5]
set_1 = {number*2 for number in numbers}
print(set_1)
# Output: {2, 4, 6, 8, 10}

If you have some cool tricks regarding this topic, feel free to comment below. Thanks.