title: Matrix addition in python
body: <p>I am trying to write simple function that will return sum of two matrices (nested lists). I do not understand why the code below does not work as expected. Note that all printed steps are fine, but the result at the end is not.</p>
  <p>Code:</p>
  <pre><code>matrix_a = [[1, 2, 4], [0, 1, 3], [2, 2, 8]]
  matrix_b = [[0, 3, 1], [5, 4, 2], [8, 2, 6]]
  
  rows = len(matrix_a)
  columns = len(matrix_a[0])
  result = [[0] * columns] * rows
  for x in range(rows):
   for y in range(columns):
     result[x][y] = matrix_a[x][y] + matrix_b[x][y]
     print(f'[{x}][{y}]: {matrix_a[x][y]} + {matrix_b[x][y]} = {result[x][y]}')
  print(result)
  </code></pre>
  <p>Output:</p>
  <pre><code>[0][0]: 1 + 0 = 1
  [0][1]: 2 + 3 = 5
  [0][2]: 4 + 1 = 5
  [1][0]: 0 + 5 = 5
  [1][1]: 1 + 4 = 5
  [1][2]: 3 + 2 = 5
  [2][0]: 2 + 8 = 10
  [2][1]: 2 + 2 = 4
  [2][2]: 8 + 6 = 14
  [[10, 4, 14], [10, 4, 14], [10, 4, 14]]
  </code></pre>
  <p>Could you tell my how to fix it and, what is more important for me, why last row is copied three times?</p>