Работа с матрицами

class Matrix:
    def __init__(self, L):
        for i in L:
            if not isinstance(i, list):
                raise ValueError('Wrong input')
            for j in i:
                if type(j) not in (int, float):
                    raise ValueError('Wrong input')
        self.matrix = L
 
    def __str__(self):
        return '['+'\n '.join(map(str, self.matrix))+']'
 
    def __repr__(self):
        return self.__str__()
 
    def __add__(self, other):
        if isinstance(other, Matrix):
            res = []
            for i in range(len(self.matrix)):
                try:
                    res.append([self.matrix[i][j] + other.matrix[i][j] for j in range(len(self.matrix))])
                except IndexError:
                    raise ValueError('Matrices must have the form n * n and / or be of the same dimension')
            return Matrix(res)
 
    def __sub__(self, other):
        if isinstance(other, Matrix):
            res = []
            for i in range(len(self.matrix)):
                try:
                    res.append([self.matrix[i][j] - other.matrix[i][j] for j in range(len(self.matrix))])
                except IndexError:
                    raise ValueError('Matrices must have the form n * n and / or be of the same dimension')
            return Matrix(res)
 
 
m1 = Matrix([[1, 2, 3], [4, 5, 6]])
m2 = Matrix([[7, 8, 9], [10, 11, 12]])
m3 = m1 + m2
m4 = m1 - m2
print(m1)
print(m2)
print(m3)
print(m4)

for j in range(len(self.matrix))])

for j in range(len(self.matrix[0]))])