英文文档：
class slice(stop) class slice(start, stop[, step]) Return a slice object representing the set of indices specified by range(start, stop, step). The start and step arguments default to None. Slice objects have read-only data attributes start, stop and step which merely return the argument values (or their default). They have no other explicit functionality; however they are used by Numerical Python and other third party extensions. Slice objects are also generated when extended indexing syntax is used. For example: a[start:stop:step] or a[start:stop, i]. See itertools.islice() for an alternate version that returns an iterator.
说明：
1. 函数实际上是一个切片类的构造函数，返回一个切片对象。
2. 切片对象由3个属性start、stop、step组成，start和step默认值为None。切片对象主要用于对序列对象进行切片取对应元素。
>>> help(slice)
class slice(object)
| slice(stop)
| slice(start, stop[, step])
|
| Create a slice object. This is used for extended slicing (e.g. a[0:10:2]).
|
| Methods defined here:
|
| ...#省略#
| ----------------------------------------------------------------------
| Data descriptors defined here:
|
| start
|
| step
|
| stop
|
| ----------------------------------------------------------------------
| Data and other attributes defined here:
|
| __hash__ = None
>>> a = list(range(10))
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a[None:5:None] # start step显式为None
[0, 1, 2, 3, 4]
>>> a[:5:] # start step默认为None
[0, 1, 2, 3, 4]
>>> a[2:5:None] # step显式为None
[2, 3, 4]
>>> a[2:5:] # step默认为None
[2, 3, 4]
>>> a[1:10:3]
[1, 4, 7]
3. 对应切片对象的3个属性start、stop、step，slice函数也有3个对应的参数start、stop、step，其值分别会付给切片对象的start、stop、step。
>>> c1 = slice(5) # 定义c1
>>> c1
slice(None, 5, None)
>>> c2 = slice(2,5) # 定义c2
>>> c2
slice(2, 5, None)
>>> c3 = slice(1,10,3) # 定义c3
>>> c3
slice(1, 10, 3)
>>> a[c1] # 和a[:5:]结果相同
[0, 1, 2, 3, 4]
>>> a[c2] # 和a[2:5:]结果相同
[2, 3, 4]
>>> a[c3] # 和a[1:10:3]结果相同
[1, 4, 7]
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