Python内置函数(19)-slice

官方文档

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.函数实际上是切片类的一个构造函数，返回一个切片对象

         2.切片对象由3个参数组成，start、stop、step组成，start和step默认是None。切片对象主要是对序列对象进行切片取元素

>>> help(slice)

 |      Return repr(self).
 |  
 |  indices(...)
 |      S.indices(len) -> (start, stop, stride)
 |      
 |      Assuming a sequence of length len, calculate the start and stop
 |      indices, and the stride length of the extended slice described by
 |      S. Out of bounds indices are clipped in a manner consistent with the
 |      handling of normal slices.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  start
 |  
 |  step
 |  
 |  stop
 |  
 |  ----------------------------------------------------------------------
 |  Data and other attributes defined here:
 |  
 |  __hash__ = None

li = [11,22,33,44,55,66,77]

ret1 = li[None:5:None]  #start,stem为None
ret2 = li[:5:] #同上
print(ret1,ret2)
#输出
#[11, 22, 33, 44, 55]  [11, 22, 33, 44, 55]

ret3 = li[2:5:None] #step为None
ret4 = li[2:5:]
print(ret3,ret4)    #同上
#输出
#[33, 44, 55]
#[33, 44, 55]

ret5 = li[1:6:3]
print(ret5)
#输出
#[22, 55]

　　3.对应切片的3个属性start、stop、step,slice函数也有3个对应的参数start、stop、step,其值会直接赋给切片对象的start、stop、step

#定义c1
c1 = slice(5)
print(c1)
#输出
#slice(None, 5, None)

#定义c2
c2 = slice(2,5)
print(c2)
#输出
#slice(2, 5, None)

#定义c3
c3 = slice(1,6,3)
print(c3)
#输出
#slice(1, 6, 3)

a = list(range(1,10))
ret1 = a[c1]   #同a[:5:]一样
print(ret1)
#输出
#[1, 2, 3, 4, 5]

ret2 = a[c2]  #同a[2:5:]一样
print(ret2)
#输出
#[3, 4, 5]

ret3 = a[c3]  #同a[1:6:3]一样
print(ret3)
#输出
#[2, 5]