Three Elegant Python Tricks to Sum Columns in a 2D List

The author, a Python enthusiast, shares a question from a fan about summing columns of a 2D list.

1. Problem Statement

A list of lists is given, and the goal is to compute the sum of each column without manually defining a variable for each element.

lst = [[1, 2, 3, 4], [1, 5, 1, 2], [2, 3, 4, 5], [5, 3, 1, 3]
s1 = 0
s2 = 0
s3 = 0
s4 = 0
for i in lst:
    s1 += i[0]
    s2 += i[1]
    s3 += i[2]
    s4 += i[3]
print(list([s1, s2, s3, s4]))

The above works but is not scalable for many columns.

2. Solutions

Solution by Yuliang

lst = [[1, 2, 3, 4], [1, 5, 1, 2], [2, 3, 4, 5], [5, 3, 1, 3]]
[print(sum(i)) for i in zip(*lst)]

This uses list unpacking with zip to sum each column.

Solution by Daler (using NumPy)

import numpy as np

lst = [[1, 2, 3, 4], [1, 5, 1, 2], [2, 3, 4, 5], [5, 3, 1, 3]]

list1 = np.sum(lst, axis=0)  # column-wise sum
list2 = np.sum(lst, axis=1)  # row-wise sum
print(list1)
print(list2)

NumPy provides concise functions for both column and row sums.

Solution by Moon God

from functools import reduce

lst = [[1, 2, 3, 4],
       [1, 5, 1, 2, 6],
       [2, 3, 4, 5],
       [5, 3, 1, 3]]

print(list(reduce(lambda x, y: map(lambda i, j: i + j, x, y), lst)))

This approach uses reduce and map with a lambda to aggregate column sums.

3. Conclusion

The article presented three different methods to sum corresponding elements of sub‑lists in a 2D list, demonstrating plain Python loops, list unpacking with zip, NumPy operations, and functional programming with reduce. Readers are encouraged to try alternative solutions.