title: I create a python list and cannot get expected output, how to fix it?
body: <p>code:</p>
  <pre><code>ans = [set()] * 3
  ans[1].add('s')
  print(ans)
  </code></pre>
  <p>output:</p>
  <pre><code>[{'s'}, {'s'}, {'s'}]
  </code></pre>
  <p>my expected output is <code>[{},{'s'},{}]</code></p>
  <p>code:</p>
  <pre><code>ans = [[]] * 3
  ans[1].append('s')
  print(ans)
  </code></pre>
  <p>output:</p>
  <pre><code>[['s'], ['s'], ['s']]
  </code></pre>
  <p>my expected output is <code>[[],['s'],[]]</code></p>
  <p>WHY??</p>