Mean of combined sublists with different lengths
Assuming your nesting is only one level deep, the concatenation can be done very easily using np.hstack.  This will treat the inner lists as vectors, then concatenate them end-to-end as a 1D numpy array.  You can then take the mean of the resulting array.  So this will do what you want:

&gt;&gt;&gt; np.hstack(a).mean()
10.79


Or equivalently (but more verbose in my opinion):

&gt;&gt;&gt; np.mean(np.hstack(a))
10.79