simple minesweeper operations in python3

minesweeper.py

from random import randint
from typing import List


class Board:
    def __init__(self, length: int, width: int, bombs: int) -> List:
        self.length = length
        self.width = width
        self.board = []
        for i in range(length):
            self.board.append([0] * width)
        for _ in range(bombs):
            while True:
                x = randint(0, length - 1)
                y = randint(0, width - 1)
                if self.board[x][y] == 0:
                    self.board[x][y] = 'x'
                    break
        self.setup_hints()

    def __repr__(self) -> str:
        r = ''
        for i in range(self.length):
            r += ''.join((f'{i:^5}' for i in self.board[i]))
            r += '\n'
        return r

    def setup_hints(self) -> None:
        board = self.board
        for row in range(self.length):
            for col in range(self.width):
                if board[row][col] != 'x':
                    continue
                self.update_hints(row, col)

    def update_hints(self, row: int, col: int) -> None:
        self.safe_update(row, col + 1)
        self.safe_update(row, col - 1)
        self.safe_update(row - 1, col)
        self.safe_update(row - 1, col - 1)
        self.safe_update(row - 1, col + 1)
        self.safe_update(row + 1, col)
        self.safe_update(row + 1, col + 1)
        self.safe_update(row + 1, col - 1)

    def safe_update(self, row: int, col: int) -> None:
        if row >= self.length or col >= self.width or row < 0 or col < 0:
            return
        if self.board[row][col] == 'x':
            return
        self.board[row][col] += 1

    def click(self,
              row: int,
              col: int,
              visited: List = None,
              cells: List = None) -> List:
        board = self.board
        if visited is None:
            visited = []
        if cells is None:
            cells = []
        if row >= self.length or col >= self.width or row < 0 or col < 0:
            return
        if board[row][col] == 'x':
            return
        cell_signature = row * 10 + col
        if cell_signature in visited:
            return

        board[row][col] = f'-{board[row][col]}-'
        cells.append((row, col))
        visited.append(cell_signature)
        self.click(row, col + 1, visited, cells)
        self.click(row, col - 1, visited, cells)
        self.click(row - 1, col, visited, cells)
        self.click(row - 1, col + 1, visited, cells)
        self.click(row - 1, col - 1, visited, cells)
        self.click(row + 1, col, visited, cells)
        self.click(row + 1, col + 1, visited, cells)
        self.click(row + 1, col - 1, visited, cells)
        return cells