Nested List Comprehensions in Python

List Comprehension are one of the most amazing features of Python. It is a smart and concise way of creating lists by iterating over an iterable object. Nested List Comprehensions are nothing but a list comprehension within another list comprehension which is quite similar to nested for loops.

Syntax of Nested List Comprehension

new_list = [[expr for item in inner_iterable] for item in outer_iterable]

Parameters:

• expr: The expression to compute or transform each item.
• inner_iterable: The iterable used for the inner list.
• outer_iterable: The iterable used for the outer list.

Let's look at some of the examples:

Example 1: Creating a Matrix

Without List Comprehension

m = []

for i in range(5):
    m.append([])               # Add empty row
    for j in range(5):
        m[i].append(j)         # Add values 0–4 in each row

print(m)

[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

Explanation: This code builds a 5x5 matrix (list of lists), where each row contains numbers from 0 to 4 using nested loops.

Using List Comprehension

m = [[c for c in range(5)] for r in range(5)]
print(m)

[[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]

Explanation: A more concise version of the same logic, the inner loop [c for c in range(5)] creates each row and the outer loop repeats it 5 times.

Example 2: Filtering a Nested List Using List Comprehension

Without List Comprehension

m = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
odds = []

for r in m:
    for e in r:
        if e % 2 != 0:       # Check if element is odd
            odds.append(e)

print(odds)

[1, 3, 5, 7, 9]

Explanation: This loops through a 2D list and adds all odd numbers to a new list odds.

Using List Comprehension

m = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

odds = [e for r in m for e in r if e % 2 != 0]
print(odds)

[1, 3, 5, 7, 9]

Explanation: This single line does the same thing, it goes through each row 'r', each element 'e' and includes 'e' only if it's odd.

Example 3: Flattening Nested Sub-Lists

Without List Comprehension

m = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]

flat = []
for s in m:
    for v in s:
        flat.append(v)

print(flat)

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Explanation: This code flattens a nested list m by iterating over each sublist and adding every value to flat.

With List Comprehension

m = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]

flat = [v for row in m for v in row]
print(flat)

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Explanation: A more compact version that loops through every sublist r and each value v and collects all values into a flat list.

Example 4: Manipulate String Using List Comprehension

Without List Comprehension

m = [["apple", "banana", "cherry"],
     ["date", "fig", "grape"],
     ["kiwi", "lemon", "mango"]]

mod_m = []
for r in m:
    mod_r = []
    for f in r:
        mod_r.append(f.capitalize())
    mod_m.append(mod_r)

print(mod_m)

[['Apple', 'Banana', 'Cherry'], ['Date', 'Fig', 'Grape'], ['Kiwi', 'Lemon', 'Mango']]

Explanation: Each string (fruit name) in the nested list is capitalized using nested loops.

With List Comprehension

m = [["apple", "banana", "cherry"],
     ["date", "fig", "grape"],
     ["kiwi", "lemon", "mango"]]

mod_m = [[f.capitalize() for f in r] for r in m]
print(mod_m)

[['Apple', 'Banana', 'Cherry'], ['Date', 'Fig', 'Grape'], ['Kiwi', 'Lemon', 'Mango']]

Explanation: Each fruit f in every row r is capitalized using .capitalize() and the modified rows are collected into mod_m.