Python3 m+n solution

m+n.py

#!/usr/bin/env python3
import unittest


def zero(matrix):
    if len(matrix) == 0:
        return matrix

    rows = set()
    cols = set()
    m = len(matrix)
    n = len(matrix[0])

    for c in range(m):
        for r in range(n):
            if matrix[c][r] == 0:
                cols.add(c)
                rows.add(r)

    for c in cols:
        for r in range(n):
            matrix[c][r] = 0

    for r in rows:
        for c in range(m):
            matrix[c][r] = 0

    return matrix


if __name__ == '__main__':
    tc = unittest.TestCase('__init__')

    tc.assertEqual(zero([
        [1, 1, 1],
        [1, 0, 1],
        [1, 1, 1]
    ]), [
        [1, 0, 1],
        [0, 0, 0],
        [1, 0, 1]
    ])

    tc.assertEqual(zero([
        [0, 1, 2, 0],
        [3, 4, 5, 2],
        [1, 3, 1, 5]
    ]), [
        [0, 0, 0, 0],
        [0, 4, 5, 0],
        [0, 3, 1, 0]
    ])

    print('Tests pass!')