import numpy as np

arr1 = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
arr2 = np.array([[13, 14, 15], [16, 17, 18]])
matrix = [arr1, arr2]

print('Numpy stacking in a loop - intermediary matrix')
print('----------------------------------------------')
print(matrix)