Python arrays without numpy

def DiagnolSum(self, array1):
        self.array1 = array1
        for i in range(len(array1)):

This is a method i have made by making a class called Array and when i run the above code it gives me an error of object of Array has no len() function, can somebody help me with it?? Actually i am performing the sum of diagnol of a matrix but just for printing the matrix in list form, i have to run this code.
Furthermore, I am using it without numpy!!

Check if you’ve used len for an instance of the Array class.

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