Probleme boucle for

class Sudoku:

    def __init__(self, tab=[[0] * 9] * 9):
        self.size = 9
        self.tab = [n if n else [] for n in tab]
        self.cases_to_find = []

        # On remplit self.cases_to_find avec les coordonées des cases vides
for y in range(self.size):
            for x in range(self.size):
                if not self.tab[y][x]:
                    self.cases_to_find.append((x, y))
        self.possible = [] # liste qu'on vas remplir avec les valeurs possibles 
        self.found = 0
def solve(self):
        self.solve_1()
        self.solve_2()

    def solve_1(self):
        liste_found = []
        possible = []
        found = self.found

        # CETTE BOUCLE EN DESSOUS
for i, (x, y) in enumerate(self.cases_to_find):
            case = []
            for j in range(1, self.size+1):
                if j not in self._line(y) + self._column(x) + self._case(x, y):
                    case.append(j)
            if len(case) == 0:
                raise SudokuError("Invalid grid")
            elif len(case) == 1:
                logging.debug(f"case trouvee : {(x, y)} : {case[0]}")
                self.tab[y][x] = case[0]
                liste_found.append(i)
                found += 1
else:
                possible.append(tuple(case))
        for i in liste_found:
            del self.cases_to_find[i]
        if found != self.found:
            self.possible = possible
            self.solve_1()

    def solve_2(self):
        pass
# apres il y a les methodes _line, _column et _case qui renvoient des listes correspondantes en enlevant les 0 ( qui correspondent aux cases vides )