title: Appending to one list in a list of lists appends to all other lists, too
body: <p>I'm getting mad with list indexes, and can't explain what I'm doing wrong.</p>
  
  <p>I have this piece of code in which I want to create a list of lists, each one containing values of the same circuit parameter (voltage, current etc..) that I'm reading from a <code>csv</code> file that looks like this:</p>
  
  <pre><code>Sample, V1, I1, V2, I2
  0, 3, 0.01, 3, 0.02
  1, 3, 0.01, 3, 0.03
  </code></pre>
  
  <p>And so on. What I want is to create a list that for example contains V1 and I1 (but I want to chose interactively) in the form [[V1], [I1]], so:</p>
  
  <pre><code>[[3,3], [0.01, 0.01]]
  </code></pre>
  
  <p>The code that I'm using is this:</p>
  
  <pre><code>plot_data = [[]]*len(positions)    
  for row in reader:
      for place in range(len(positions)):
          value = float(row[positions[place]])
          plot_data[place].append(value)
  </code></pre>
  
  <p><code>plot_data</code> is the list that contains all the values, while <code>positions</code> is a list with the indexes of the columns that I want to copy from the <code>.csv</code> file. The problem is that if I try the commands in the shell, seems to work, but if I run the script instead of appending each value to the proper sub-list, it appends all values to all lists, so I obtain 2 (or more) identical lists.</p>