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]
