title: Python: Append a list to another list and Clear the first list
body: <p>So this just blew my mind. I am working on a python code where I create a list, append it to a master list and clear the first list to add some more elements to it. When I clear the first list, even the master list gets cleared. I worked on a lot of list appends and clears but never observed this.</p>
  
  <pre><code>list1 = []
  list2 = []
  list1 = [1,2,3]
  list2.append(list1)
  list1
  [1, 2, 3]
  list2
  [[1, 2, 3]]
  del list1[:]
  list1
  []
  list2
  [[]]
  </code></pre>
  
  <p>I know this happens with appending dictionaries but I did not know how to deal with lists. Can some one please elaborate?</p>
  
  <p>I am using Python2.7</p>