Question

I want to define class to represent a matrix

class matrix:
    def __init__(self, mat):
        self.mat = mat
        self.dim = len(mat)

    @classmethod
    def withDim(matrix, dimension):
        mat = [ [0]*dimension for i in range(dimension)]
        return matrix(mat)

where mat is a list of lists, so to represent the matrix

A = | a b |
    | c d |

I can write the following

A = matrix( [ [a, b], [c, d] ])

I’ve also started to define some operators, like

def __add__(self, other):
    n = self.dim
    result = self.withDim(n)
    for i in range(n):
        for j in range(n):
            result.mat[i][j] = self.mat[i][j] + other.mat[i][j]
    return result

Now if I want to access element i, j in matrix A I have to do

A.mat[i][j]

The question is: can I define the operator [ ] in order to do

A[i][j]

just like I defined __add__?

Answer 1

operator[] calls __getitem__:

class A:
    def __getitem__(self, index):
        return index+1

a = A()
print(a[1]) # prints 2

You can implement [][] by returning a proxy object which represents the row and also responds to __getitem__. Or, you can accept tuple as index and use A[i,j] syntax.