[ Python 常見問題 ] Understanding Python's slice notation

Source From Here 
Question 
I need a good explanation (references are a plus) on Python's slice notation. To me, this notation needs a bit of picking up. 
It looks extremely powerful, but I haven't quite got my head around it. 

How-To 
It's pretty simple really: 

view plaincopy to clipboardprint?

1. a[start:end] # items start through end-1  
2. a[start:]    # items start through the rest of the array  
3. a[:end]      # items from the beginning through end-1  
4. a[:]         # a copy of the whole array  

There is also the step value, which can be used with any of the above: 

view plaincopy to clipboardprint?

1. a[start:end:step] # start through not past end, by step  

The key point to remember is that the :end value represents the first value that is not in the selected slice. So, the difference between end and start is the number of elements selected (if step is 1, the default). The other feature is that startor end may be a negative number, which means it counts from the end of the array instead of the beginning. So: 

view plaincopy to clipboardprint?

1. a[-1]    # last item in the array  
2. a[-2:]   # last two items in the array  
3. a[:-2]   # everything except the last two items  

Similarly, step may be a negative number: 

view plaincopy to clipboardprint?

1. a[::-1]    # all items in the array, reversed  
2. a[1::-1]   # the first two items, reversed  
3. a[:-3:-1]  # the last two items, reversed  
4. a[-3::-1]  # everything except the last two items, reversed  

Let's take a look at real example: 

>>> alist = list(range(10))
>>> alist
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> alist[3:] # Skip first 3 items
[3, 4, 5, 6, 7, 8, 9]
>>> alist[:-3] # Skip last 3 items
[0, 1, 2, 3, 4, 5, 6]
>>> alist[::-1] # Reverse the list
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> alist[:-3:-1] # Inverse the list and then select element until meet '7'
[9, 8]
>>> alist[-3::-1] # Inverse the list and start from '7'
[7, 6, 5, 4, 3, 2, 1, 0]

Supplement 
* What does list[x::y] do? [duplicate]