How to get the remaining indexes after a random set of indexes is selected in Python?

Use the following code to illustrate my question.

import numpy as np
np.random.seed(200)
a = np.array([1,21,6,41,8]) # given an array with 5 elements
idx = np.random.choice(5, 3, replace=False) # randomly select 3 indexes between 0 and 4
idx.sort() # sort indexes
print(idx) # [0 3 4]
print(a[idx]) # get random selected subset using the indexes, [ 1 41  8]

How to get the remaining indexes [1,2]?