Zero Matrix gist

"""
Write an algorithm such that if an element in an MxN matrix is 0, its entire row and column are set to 0.
M = width
N = height
"""

def zero_matrix(matrix):
  zero_cols = set()
  zero_rows = set()
  for lst in matrix:
    for num in range(len(lst)):
      if lst[num] == 0:
        zero_cols.add(num)
        zero_rows.add(matrix.index(lst))
  for lst in range(len(matrix)):
    if lst in zero_rows:
      matrix[lst] = [0 for i in range(len(matrix[0]))]
  for lst in matrix:
    for num in range(len(lst)):
      if num in zero_cols:
        for lst in matrix:
          lst[num] = 0
  return matrix