title: Python: Elementwise join of two lists of same length
body: <p>I have two lists of the same length </p>
  
  <pre><code>a = [[1,2], [2,3], [3,4]]
  b = [[9], [10,11], [12,13,19,20]]
  </code></pre>
  
  <p>and want to combine them to</p>
  
  <pre><code>c = [[1, 2, 9], [2, 3, 10, 11], [3, 4, 12, 13, 19, 20]]
  </code></pre>
  
  <p>I do this by</p>
  
  <pre><code>c= []
  for i in range(0,len(a)):
      c.append(a[i]+ b[i])
  </code></pre>
  
  <p>However, I am used from R to avoid for loops and the alternatives like zip and itertools do not generate my desired output. Is there a way to do it better? </p>
  
  <p><strong>EDIT:</strong> 
  Thanks for the help! My lists have 300,000 components. The execution time of the solutions are</p>
  
  <pre><code>[a_ + b_ for a_, b_ in zip(a, b)] 
  1.59425 seconds
  list(map(operator.add, a, b))
  2.11901 seconds
  </code></pre>