Ndarray array slicing problem in Python a[-n-x:-y]

First look at the following code:

>>a=np.arange (10)
>>a
Array ([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>A[-7]
Array ([3, 4, 5, 6, 7, 8, 9])
>>A[-7-1:-1]
Array ([2, 3, 4, 5, 6, 7, 8])
>>a[-7-1:-2]
Array ([2, 3, 4, 5, 6, 7])
>>A[-7-2:-1]
Array ([1, 2, 3, 4, 5, 6, 7, 8])
>> A[-7-2:]
Array ([1, 2, 3, 4, 5, 6, 7, 8, 9])

As can be seen from the above example, A[-n] is to select the last n elements of a array to form an array. A[-N-X:-Y] indicates that x elements are taken forward on the formed A[-n] array, and that the Y element is less than the backward one. Like what:

A[-7-1:-2]=[2, 3, 4, 5, 6, 7], this is the A[-7]=[3, 4, 5, 6, 7, 8, 9] on the basis of a number of forward more than a few 2, backward take two number 8, 9, the formation of a new array.

Ndarray array slicing problem in Python a[-n-x:-y]