title: Python/Numpy subarray selection
body: <p>I have some Numpy code which I'm trying to decipher. There's a line <code>v1 = v1[:, a1.tolist()]</code> which passes a numpy array <code>a1</code> and converts it into a list. I'm confused as to what <code>v1[:, a1.tolist()]</code> actually does. I know that <code>v1</code> is now being set to a column array given from <code>v1</code> given by the selection <code>[:, a1.tolist()]</code> but what's getting selected? More precisely, what is <code>[:, a.tolist()]</code> doing?</p>