Nested List Comprehensions in Python
Last Updated :
05 Jul, 2025
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
Python
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)
Output[[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
Python
m = [[c for c in range(5)] for r in range(5)]
print(m)
Output[[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
Python
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)
Explanation: This loops through a 2D list and adds all odd numbers to a new list odds.
Using List Comprehension
Python
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)
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
Python
m = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]
flat = []
for s in m:
for v in s:
flat.append(v)
print(flat)
Output[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
Python
m = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]
flat = [v for row in m for v in row]
print(flat)
Output[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
Python
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)
Output[['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
Python
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)
Output[['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.
