title: How to take only positive values in a list in Python?
body: <p>I am wondering how I can take only positive values in a list in Python.</p>
  
  <p>For example, if I have <code>A = [1, 2, 3]</code>, it should return <code>[1, 2, 3]</code></p>
  
  <p>If I have <code>A = [-1, 2, 3]</code>, it should return <code>[2, 3]</code></p>
  
  <p>if I have <code>A = [-1, -2]</code>, it should return <code>None</code></p>
  
  <p>Thank you very much!</p>