title: changin an item in a python 2-dim list will change the whole column
body: <p>I,m new in python and have a code like this in python 3.9:</p>
  <pre><code>a = [[0]*3]*3
  a[0][0] = 1
  print(a)
  </code></pre>
  <p><strong>output: [[1,0,0],[1,0,0],[1,0,0]]</strong></p>
  <p>I thought that the output should be:</p>
  <p><strong>output: [[1,0,0],[0,0,0],[0,0,0]]</strong></p>
  <p>changing this line</p>
  <pre><code>a = [[0]*3]*3
  </code></pre>
  <p>to</p>
  <pre><code>a=[]
  for i in range(3):
     a.append([0]*3)
  </code></pre>
  <p>will solve the issue</p>
  <p>could someone tell me where I am wrong please.</p>