title: python mean of list of lists
body: <p>I want to find the means of all the negative numbers from a list that has a mix of positive and negative numbers. I can find the mean of the lists as </p>
  
  <pre><code>import numpy as np
  
  listA = [ [2,3,-7,-4] , [-2,3,4,-5] , [-5,-6,-8,2] , [9,5,13,2]  ]
  listofmeans = [np.mean(i) for i in listA ]
  </code></pre>
  
  <p>I want to create a similar one line code that only takes the mean of the negative numbers in the list. So for example the first element of the new list would be (-7 + -4)/2 = -5.5</p>
  
  <p>My complete list would be:</p>
  
  <pre><code>listofnegativemeans = [ -5.5, -3.5, -6.333333, 0 ] 
  </code></pre>