It's pretty simple really:
(真的很简单：)
a[start:stop] # items start through stop-1
a[start:] # items start through the rest of the array
a[:stop] # items from the beginning through stop-1
a[:] # a copy of the whole array
There is also the step value, which can be used with any of the above:
(还有step值，可以与以上任何一个一起使用：)
a[start:stop:step] # start through not past stop, by step
The key point to remember is that the :stop value represents the first value that is not in the selected slice.
(要记住的关键点是:stop值表示不在所选切片中的第一个值。)
So, the difference between stop and start is the number of elements selected (if step is 1, the default). (因此， stop和start之间的差异是所选元素的数量（如果step为1，则为默认值）。)
The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning.
(另一个功能是start或stop可以是负数 ，这意味着它是从数组的末尾而不是开头开始计数。)
So: (所以：)
a[-1] # last item in the array
a[-2:] # last two items in the array
a[:-2] # everything except the last two items
Similarly, step may be a negative number:
(同样， step可以为负数：)
a[::-1] # all items in the array, reversed
a[1::-1] # the first two items, reversed
a[:-3:-1] # the last two items, reversed
a[-3::-1] # everything except the last two items, reversed
Python is kind to the programmer if there are fewer items than you ask for.
(如果项目数量少于您的要求，Python对程序员很友好。)
For example, if you ask for a[:-2] and a only contains one element, you get an empty list instead of an error. (例如，如果您要求a[:-2]并且a仅包含一个元素，则会得到一个空列表而不是错误。)
Sometimes you would prefer the error, so you have to be aware that this may happen. (有时您会更喜欢该错误，因此您必须意识到这种情况可能会发生。)
Relation to slice() object (与slice()对象的关系)
The slicing operator [] is actually being used in the above code with a slice() object using the : notation (which is only valid within [] ), ie:
(在上面的代码中，实际上使用:表示法将切片运算符[]与slice()对象一起使用（仅在[]有效），即：)
a[start:stop:step]
is equivalent to:
(等效于：)
a[slice(start, stop, step)]
Slice objects also behave slightly differently depending on the number of arguments, similarly to range() , ie both slice(stop) and slice(start, stop[, step]) are supported.
(切片对象的行为也取决于参数的数量，与range()略有不同，即，都支持slice(stop)和slice(start, stop[, step]) 。)
To skip specifying a given argument, one might use None , so that eg a[start:] is equivalent to a[slice(start, None)] or a[::-1] is equivalent to a[slice(None, None, -1)] . (要跳过指定给定参数的操作，可以使用None ，例如a[start:]等同于a[slice(start, None)]或a[::-1]等同于a[slice(None, None, -1)] 。)
While the : -based notation is very helpful for simple slicing, the explicit use of slice() objects simplifies the programmatic generation of slicing.
(尽管基于:的符号对于简单切片非常有用，但是对slice()对象的显式使用简化了切片的程序化生成。)
