title: zero padding numpy array
body: <p>Suppose I have a list contains un-equal length lists. </p>
  
  <pre><code>a = [ [ 1, 2, 3], [2], [2, 4] ]
  </code></pre>
  
  <p>What is the best way to obtain a zero padding numpy array with standard shape? </p>
  
  <pre><code>zero_a = [ [1, 2, 3], [2, 0, 0], [2, 4, 0] ]
  </code></pre>
  
  <p>I know I can use list operation like</p>
  
  <pre><code>n = max( map( len, a ) )
  map( lambda x : x.extend( [0] * (n-len(x)) ), a )
  zero_a = np.array(zero_a)
  </code></pre>
  
  <p>but I was wondering is there any easy numpy way to do this work?</p>