Python中的"切片列表"和"省略号"; 使用切片列表切割列表和列表

Question

原始问题:有人可以告诉我如何使用"切片列表"和"省略号"吗？它们什么时候有用？谢谢.

这是语言定义所说的关于"slice_list"和"省略号"的内容; Alex Martelli的答案指出了他们的起源,这不是我想象的.

[ http://docs.python.org/reference/expressions.html#tok-slicing][1]

5.3.3.Slicings

extended_slicing :: = primary"["slice_list"]"

slice_list :: = slice_item(","slice_item)*[","]

slice_item :: = expression | proper_slice | 省略

省略号:: ="..."

[1]:http: //docs.python.org/reference/expressions.html#tok-slicing

如果有人(像我一样)正在寻找使用切片列表攻击列表(或列表列表)的方法,这里有5种方法可以从列表中获取由切片列表选择的元素列表以及2种对列表列表执行相同操作的方法,在这种情况下,每个列表应用一个切片.输出在最后的注释中.我发现h5是一个使用嵌套for循环的例子,如果没有使用(更新)有意义的变量名,最难理解.

#!/usr/bin/env python



import itertools

puz = [(i + 100) for i in range(40)]
puz1 = list( puz)
puz2 = [(i + 200) for i in range(40)]
puz3 = [(i + 300) for i in range(40)]
puzs = [puz1,puz2,puz3]

sa = slice( 0,1,1)
sb = slice( 30,39,4)
sc = slice( -1, -15,-5)
ss = [sa,sb,sc]

def mapfunc( a,b):
    return a[b]

f = map( mapfunc,[puz] * len(ss),ss)
print "f =  ", f  #same as g below

g = [ puz[i]
    for i in ss ]
print "g =  ",g  #same as f, above

h1 = [ i 
    for i in itertools.chain( puz[sa],puz[sb],puz[sc]) ]
print "h1 = ", h1  #right 

h2 = [ i
    for i in itertools.chain( *(map( mapfunc,[puz] * len(ss),ss))) ]
print "h2 = ",h2  #right

h3 = [ i
    for i in itertools.chain( *f) ]
print "h3 = ",h3  #right

h4 = [ i 
    for i in itertools.chain( *g) ]
print "h4 = ", h4 #also right

h5 = []
for slice_object in ss:
    for list_element in puz[slice_object]:
        h5.append( list_element)
print "h5 = ", h5  #right, too

print "=============================="

hh1 = [ i
    for i in itertools.chain( *(map( mapfunc,puzs,ss))) ]
print "hh1 =  ",hh1  #right

puz_s_pairs = zip( puzs,ss)
#print "puz_s_pairs = ",puz_s_pairs
hh2 = [ i
    for i in itertools.chain( *(map( mapfunc,*zip( *puz_s_pairs)))) ]
print "hh2 =  ",hh2  #right

'''
>>> execfile(r'D:/cygwin/home/usr01/wrk/py/pyexpts/list_of_slices_of_list.02.py')
f =   [[100], [130, 134, 138], [139, 134, 129]]
g =   [[100], [130, 134, 138], [139, 134, 129]]
h1 =  [100, 130, 134, 138, 139, 134, 129]
h2 =  [100, 130, 134, 138, 139, 134, 129]
h3 =  [100, 130, 134, 138, 139, 134, 129]
h4 =  [100, 130, 134, 138, 139, 134, 129]
h5 =  [100, 130, 134, 138, 139, 134, 129]
==============================
hh1 =   [100, 230, 234, 238, 339, 334, 329]
hh2 =   [100, 230, 234, 238, 339, 334, 329]
'''

Answer 1

最初在Python中引入了切片列表和省略号,以便为numpy(良好的旧数字)的前端提供良好的语法糖.如果你正在使用numpy(没有理由回到它的任何前辈! - )你当然应该使用它们; 如果出于某种奇怪的原因,你做你自己的实现超灵活的多维数组,你肯定会想学习numpy的使用它们,可能模仿它紧密(它的方式是设计得相当不错毕竟).我想不出多维数组之外的好用途.