title: Put numpy arrays split with np.split() back together
body: <p>I have split a numpy array like so: </p>
  
  <pre><code>x = np.random.randn(10,3)
  x_split = np.split(x,5)
  </code></pre>
  
  <p>which splits x equally into five numpy arrays each with shape (2,3) and puts them in a list. What is the best way to combine a subset of these back together (e.g. <code>x_split[:k]</code> and <code>x_split[k+1:]</code>) so that the resulting shape is similar to the original x i.e. (something,3)? </p>
  
  <p>I found that for k > 0 this is possible with you do:</p>
  
  <pre><code>np.vstack((np.vstack(x_split[:k]),np.vstack(x_split[k+1:])))
  </code></pre>
  
  <p>but this does not work when k = 0 as <code>x_split[:0] = []</code> so there must be a better and cleaner way. The error message I get when k = 0 is: </p>
  
  <blockquote>
    <p>ValueError: need at least one array to concatenate</p>
  </blockquote>