function changing input value

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The following code is supposed to implement a basic percolation process on a matrix of 0's and 1's. For some reason, it's changing the value of the matrix I input, and I can't figure out why. I'm sure it's a basic error on my part, but if someone can set me straight, I'd appreciate it!

Here's the function I defined:

n = 4;
def percolate(B):
    update = B;
    for i in [0..n-1]:
        for j in [0..n-1]:
            nbr_sum = 0;
            if (i==0):
                nbr_sum += B[i+1,j];
            elif (i==n-1):
                nbr_sum += B[i-1,j];
            else:
                nbr_sum += B[i-1,j]+B[i+1,j];
            if (j==0):
                nbr_sum += B[i,j+1];
            elif (j==n-1):
                nbr_sum += B[i,j-1];
            else:
                nbr_sum += B[i,j-1]+B[i,j+1];
            if (nbr_sum > 1):
                update[i,j] = 1;
    return update;

When I execute

Bid = matrix.identity(n);
percolate(Bid)

I get the expected output

[1 1 0 0]
[1 1 1 0]
[0 1 1 1]
[0 0 1 1]

but the value of Bid also changes to the same value (as opposed to continuing to be the n x n identity matrix, as desired).