Suppose in Scipy I have a 2x2 array A and I want to switch the first two columns. I incorrectly expected the following to work: A[:,0],A[:,1]=A[:,1],A[:,0] Then I discovered this works: A=mat(A) A[:,0],A[:,1]=A[:,1],A[:,0] Question: Why the difference? And: how to anticipate this difference? Thank you, Alan Isaac