Python TypeError: 'float' object is not writable

Hello.

Using Python, we are writing a code to obtain the slope of the neural network loss function.

Here's the code.

def num_grad(self, f, key):
        h = 0.001
        W = self.params[key]

        result = [[0 for j in range(len(W[0]))] for i in range(len(W))]

        for i in range(len(W)):
            for j in range(len(W[0])):
                tmp_val = W[i][j]

                self.params[key][i][j] = tmp_val + h
                f1 = f(self.params[key])

                self.params[key][i][j] = tmp_val - h
                f2 = f(self.params[key])

                result[i][j] = (f1-f2)/(2*h)

                self.params[key] = tmp_val

        return result

At this time, self.The parameters [key] are two-dimensional list indicating weights or biases self.If you try to access the parameters [key][i][j] , the value appears TypeError: Cannot proceed with 'float' object is not writable error.

I looked it up on the Internet and found that it is an error that appears when accessing variables other than list as indexes In my case, self normally.The corresponding error appears at the same time as the value of param[key][i][j] appears.

What should I do?