my_array.sort sorts the list "in place":

Instead of returning a sorted list, as you seem to expect, my_array.sort() sorts my_array "in place" and doesn't return anything. Try this

my_array = [5, 2, 8, 1, 9]
my_array.sort()
print(my_array)  # [1, 2, 5, 8, 9]

If you want it sorted in descending order use this:

my_array = [5, 2, 8, 1, 9]
my_array.sort()
my_array.reverse()
print(my_array)  # [9, 8, 5, 2, 1]

temp[::-1].sort() sorts the array in place, whereas np.sort(temp)[::-1] creates a new array.

In [25]: temp = np.random.randint(1,10, 10)

In [26]: temp
Out[26]: array([5, 2, 7, 4, 4, 2, 8, 6, 4, 4])

In [27]: id(temp)
Out[27]: 139962713524944

In [28]: temp[::-1].sort()

In [29]: temp
Out[29]: array([8, 7, 6, 5, 4, 4, 4, 4, 2, 2])

In [30]: id(temp)
Out[30]: 139962713524944