英文文檔：

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.

說明：

　　1. 函數(shù)實(shí)際上是一個(gè)切片類的構(gòu)造函數(shù)，返回一個(gè)切片對(duì)象。

　　2. 切片對(duì)象由3個(gè)屬性start、stop、step組成，start和step默認(rèn)值為None。切片對(duì)象主要用于對(duì)序列對(duì)象進(jìn)行切片取對(duì)應(yīng)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顯式為None
[0, 1, 2, 3, 4]
>>> a[:5:] # start step默認(rèn)為None
[0, 1, 2, 3, 4]
>>> a[2:5:None] # step顯式為None
[2, 3, 4]
>>> a[2:5:] # step默認(rèn)為None
[2, 3, 4]
>>> a[1:10:3]
[1, 4, 7]

　　3. 對(duì)應(yīng)切片對(duì)象的3個(gè)屬性start、stop、step，slice函數(shù)也有3個(gè)對(duì)應(yīng)的參數(shù)start、stop、step，其值分別會(huì)付給切片對(duì)象的start、stop、step。

>>> c1 = slice(5) # 定義c1
>>> c1
slice(None, 5, None)
>>> c2 = slice(2,5) # 定義c2
>>> c2
slice(2, 5, None)
>>> c3 = slice(1,10,3) # 定義c3
>>> c3
slice(1, 10, 3)
>>> a[c1] # 和a[:5:]結(jié)果相同
[0, 1, 2, 3, 4]
>>> a[c2] # 和a[2:5:]結(jié)果相同
[2, 3, 4]
>>> a[c3] # 和a[1:10:3]結(jié)果相同
[1, 4, 7]

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