title: Merging two list with certain condition
body: <p>I have <code>listA = [2,0,0,5,6,0]</code> and <code>listB = [4,5,7,3,2,1]</code>. I want to merge these two list and get <code>listC = [2,5,7,5,6,1]</code>. So basically just copy <code>listA</code> but if an element in <code>listA</code> is zero, replace that element with element from <code>listB</code></p>
  <p>I tried using <code>for</code> and <code>if</code> loop but had no luck:</p>
  <pre><code>listC = []
  for i in listA and listB:
      if listA[i] == 0: 
          listC.append(listB[i])
      else:
          listC.append(listA[i])
  </code></pre>
  <p>I am new to python, so some explanation with solutions will be appreciated.</p>