solve sudoku python program

board = [[5, 3, 0, 0, 7, 0, 0, 0, 0],
         [6, 0, 0, 1, 9, 5, 0, 0, 0],
         [0, 9, 8, 0, 0, 0, 0, 6, 0],
         [8, 0, 0, 0, 6, 0, 0, 0, 3],
         [4, 0, 0, 8, 0, 3, 0, 0, 1],
         [7, 0, 0, 0, 2, 0, 0, 0, 6],
         [0, 6, 0, 0, 0, 0, 2, 8, 0],
         [0, 0, 0, 4, 1, 9, 0, 0, 5],
         [0, 0, 0, 0, 8, 0, 0, 7, 9]]

#import copy is for allowing to make copy of board
import copy
def print_matrix(m):
  #deepcopy is for matrices
  m2 = copy.deepcopy(m)
  print('SUDOKU:')
  line = '-'*25
  #check if empty
  if not m2:
    print('empty')
  #check every element
  for i in range(len(m2)):
    #check if modulo 3 to print line
    if i%3 == 0:
      print(line)
    line_to_print = ''
    for j in range(len(m2[0])):
      if m2[i][j] == 0:
        m2[i][j] = ' '
      #check modulo 3 to add vertical line
      if j%3 == 0:
        line_to_print += '| ' + str(m2[i][j]) + ' '
      else:
        line_to_print += str(m2[i][j]) + ' '
    #print final vertical line and horisontal line
    print(line_to_print+'|')
  print(line)

def find_zero(board):
  where_zero = []
  #check every element of board
  for i in range(len(board)):
    for j in range(len(board[0])):
      if board[i][j] == 0:
        #append the row to list
        where_zero.append(i)
        #append the colum to list
        where_zero.append(j)
        return where_zero
  return where_zero

def is_valid(board,row,col,value):
  # check rows
  if value in board[row]:
    return False
  # check colum
  for i in range(len(board)):
    if board[i][col] == value:
      return False
  # check 3*3 board
  new_row = row//3
  new_col = col//3
  # divided by 3 with 2 '//' for exact number
  for i in range(3):
    for j in range(3):
      if board[3*new_row+i][3*new_col+j] == value:
        return False
  return True

def solve(board):
  #check if empty
  if find_zero(board) == []:
    return board
  else:
    #check every number from 1 to 9
    for i in range(1,10):
      row = find_zero(board)[0]
      col = find_zero(board)[1]
      if is_valid(board,row,col,i):
        board[row][col] = i
        if solve(board) == None:
          board[row][col] = 0
        else:
          return solve(board)

#print unsoved SUDOKU
print_matrix(board)
#print solved SUDOKU
print_matrix(solve(board))