How to solve multiple sudokus in one file with python?

I am making a sudoku solver for school and one part of it, is that it needs to be able to solve multiple sudokus in a file. Currently it can solve one at a time, but how would I code it to solve multiple?

board = []
#open the file with puzzles in it
with open('project.txt','r') as file:
    for line in file:
         board.append(list(map(int,line.strip())))
    

#check puzzle for empty spaces
def solve(puzzle):
    find = find_empty(puzzle)
    if not find:
        return True
    else:
        row, col = find
    #go through 1 to 9 for each empty space
    for vertical in range(1,10):
        if valid(puzzle, vertical, (row, col)):
            puzzle[row][col] = vertical
            #continue to next empty space if number works
            if solve(puzzle):
                return True
            #backtrack until correct number is found
            puzzle[row][col] = 0

    return False


def valid(puzzle, num, pos):
    # Check each vertical row for correct numbers
    for vertical in range(len(puzzle[0])):
        if puzzle[pos[0]][vertical] == num and pos[1] != vertical:
            return False

    # Check each horizontal column for correct numbers
    for vertical in range(len(puzzle)):
        if puzzle[vertical][pos[1]] == num and pos[0] != vertical:
            return False

    # Check each 3x3 box for correct numbers
    box_x = pos[1] // 3
    box_y = pos[0] // 3

    for vertical in range(box_y*3, box_y*3 + 3):
        for horizontal in range(box_x * 3, box_x*3 + 3):
            #if any wrong numbers appear try again
            if puzzle[vertical][horizontal] == num and (vertical,horizontal) != pos:
                return False
    #if all numbers are correct solve puzzle
    return True

#formatting for how the final board should look
def print_board(puzzle):
    #open a new file for the answers to go into
    with open('files.txt', 'w') as file:
        #each row should have this formatting
        for vertical in range(len(puzzle)):
            if vertical % 3 == 0 and vertical != 0:
                print("- - - - - - - - - - - - - ", file=file)

            #each column should have this formatting
            for horizontal in range(len(puzzle[0])):
                if horizontal % 3 == 0 and horizontal != 0:
                    print(" | ", end="", file=file)

                if horizontal == 8:
                    print(puzzle[vertical][horizontal], file=file)
                else:
                    print(str(puzzle[vertical][horizontal]) + " ", end="", file=file)


def find_empty(puzzle):
    for vertical in range(len(puzzle)):
        for horizontal in range(len(puzzle[0])):
            if puzzle[vertical][horizontal] == 0:
                return (vertical, horizontal)  # row, col

    return None

#print the final solved puzzles 
solve(board)
print_board(board)

the file would be set up like this:

and we don't know how many puzzles there will be in the end. Thanks for any help!

Solution

The problem was that you were putting all the puzzles into a single board. Simply use the empty line (\n) between the puzzles as a delimiter to separate the board.

Besides that, you should use if __name__ == "__main__": to

protects users from accidentally invoking the script when they didn't intend to.

if __name__ == "__main__":
    board_list = []
    board = []
    # open the file with puzzles in it
    with open('project.txt', 'r') as file:
        for line in file:
            # print(line)
            if line == "\n":
                print("\\n detected")
                board_list.append(board)
                print(board_list)
                board = []
            else:
                #print("constructing board")
                board.append(list(map(int, line.strip())))
        board_list.append(board)

    for i in range(len(board_list)):
        # print the final solved puzzles
        solve(board_list[i])
        print_board(board_list[i], i)

I have also modified print_board() so that it write the puzzles into different files.

def print_board(puzzle, index):
    # take the index of the board to write it to different files
    with open(f'files{index}.txt', 'w') as file: 
        # your code