make pairwise difference of array with python

Question

I have an array with numbers like this, in this case 5 numbers:

a = array([2.88581812, 2.8930633 , 2.85603976, 2.86739916, 2.85736707])

I would like to create an array of 10 elements in which the pairwise difference of all the numbers of the array is present. For now i used nested loop for like this:

diffmean = []
for i in range(len(a)-1):
    for j in range(i+1, len(a)):
        diffmean.append(a[i]-a[j])

Obtaining a list with 10 elements of pairwise difference

    [-0.007245185215707384,
 0.029778354907735505,
 0.018418952746142025,
 0.0284510446999775,
 0.03702354012344289,
 0.02566413796184941,
 0.035696229915684885,
 -0.01135940216159348,
 -0.0013273102077580035,
 0.010032091953835476]

there is a "pythonic" way to perform this? without loopfor or nested loop for?

Answer 1

Assuming a is is numpy array. The below should work, but perhaps a more efficient solution exists as this calculates differences twices

np.expand_dims(a, 1) - a
d[np.tril_indices(a.size, k=-1)]