Sudoku object - with hints

sudukoObject.py

#Note this requires the function solve_sudoku.

def Sudoku(grid):
    def __init__(self):
        self.grid = grid
        self.solution = solve_sudoku(self.grid)
        self.hints = self.get_hints()

    def solvable(self):
        return bool(self.solution)

    def solved(self):
        if self.solvable:
            return self.grid is self.solution

    def get_hints(self):
        '''if solved or unsolvable, returns None
          else: returns list of moves (row, col, value) to complete the sudoku
        '''
        if not self.solvable() or self.solved():
            return None
        hints = []
        for row in range(9):
            for col in range(9):
                if self.grid[row][col] is 0:
                    hints.append( (row, col, self.solution[row][col]) )
        return hints

    def random_hint(self):
        '''If any more moves, returns a possible move (row, col, value)
          else: returns None
        '''
        if self.hints:
            hint = random.choice(hints)
            return hint

    def fill_in(self, row, col, value):
        '''Tries to insert value into the empty square, grid[row][col].
          if grid[row][col] is not blank: returns None
          else if inserting leads to no solutions: returns False
          else if can insert value into grid[row][col]: returns True
          else: returns None
        '''
        if not (row in range(9) and type(row) is int):
            return None
        if not (col in range(9) and type(col) is int):
            return None
        if not (value in range(1,10) and type(value) is int):
            return None
        if self.solvable() is False:
            return False
        if self.grid[row][col] is 0:
            if self.solution[row][col] is value:
                #value matches solution, so do not need to update solution
                self.grid[row][col] = value
                self.hints.remove((row,col,value))
            else:
                #update is different from solution, so need to check
                self.grid[row][col] = value
                solved = solve_solution(self.grid)
                if not solved:
                    self.grid[row][col] = 0
                    return False
                self.solution = solved
                self.hints = self.get_hints()
                return True
        return None