title: Python copy a list of lists
body: <p>I'm using python 3.4.1.<br>
  For a single list <code>a=[1,2]</code>, if I make a copy of it, <code>b = a.copy()</code> when I change items in <code>b</code>, it won't change items in <code>a</code>.<br>
  However, when I define a list of lists (actually a matrix) <code>a = [[1,2],[3,4]]</code>, when I assign <code>b = a.copy()</code>. What I do to list <code>b</code> actually affects <code>a</code>.<br>
  I checked their addresses, they are different.<br>
  Can anyone tell me why?</p>
  
  <p>ps: What I did is <code>b[0][0] = x</code>, and the item in a was also changed.</p>