Question

这个slice(-1,0)从一段实际代码中掉出来，看起来像是一个切片包装，差不多，但它不是。

评论：我怀疑是一个真正的＆＃34;切片包装＆＃34;将是另一个挑战。

在＆＃34;几乎包裹＆＃34; slice，简单的解决方法是在这种特殊情况下删除上限：

AEIOU[-1:], AEIOU[-1:None], or (more quirkilly) AEIOU[-1:5]

更普遍的尝试:(我正在使用： Python 2.6.6 ）

AEIOU[start%len(AEIOU): stop%len(AEIOU)] # simply struggles.

但鉴于负面成员函数__getitem__总能正常工作，所以有一种奇怪的（危险的）是一种近乎包裹的＆＃34;成员__getslice__只是返回太短的东西。

问题：

是否有一个特定的设计理由，即＆＃34;近包装＆＃34;切片很短，而不是（可能）提出IndexError: (list/string/tuple) index out of range" exception？
如何创建列表的子类来管理它？
Python 3.0 的处理方式有何不同？

演示：

>>> AEIOU="AEIOU"
>>> for i in range(-len(),5,1): print "%-16s, %r, %r"%(slice(i,i+1,1),AEIOU[i],AEIOU[i:i+1])
... 
slice(-5, -4, 1), 'A', 'A'
slice(-4, -3, 1), 'E', 'E'
slice(-3, -2, 1), 'I', 'I'
slice(-2, -1, 1), 'O', 'O'
slice(-1, 0, 1) , 'U', '' # missing U...
slice(0, 1, 1)  , 'A', 'A'
slice(1, 2, 1)  , 'E', 'E'
slice(2, 3, 1)  , 'I', 'I'
slice(3, 4, 1)  , 'O', 'O'
slice(4, 5, 1)  , 'U', 'U'

我注意到了一般的＆＃34;解决方法＆＃34;是：

>>> i=-1; AEIOU[i:i%len(AEIOU)+1]
'U' # found U

勘误表，切片示例：

>>> s=3
>>> for i in range(-len(AEIOU),5,1): print "%-16s, %r, %r, %r"%(slice(i,i%len(AEIOU)+s,1),AEIOU[i],AEIOU[i:i+s], AEIOU[i:i%len(AEIOU)+s])
... 
slice(-5, 3, 1) , 'A', 'AEI', 'AEI'
slice(-4, 4, 1) , 'E', 'EIO', 'EIO'
slice(-3, 5, 1) , 'I', '', 'IOU'
slice(-2, 6, 1) , 'O', '', 'OU'
slice(-1, 7, 1) , 'U', '', 'U'
slice(0, 3, 1)  , 'A', 'AEI', 'AEI'
slice(1, 4, 1)  , 'E', 'EIO', 'EIO'
slice(2, 5, 1)  , 'I', 'IOU', 'IOU'
slice(3, 6, 1)  , 'O', 'OU', 'OU'
slice(4, 7, 1)  , 'U', 'U', 'U'

Answer 1

负数指数从列表末尾开始倒数，但如果它们足够大，它们可以超过开头。当他们超调时，他们只需固定到列表中的第一项。

items = range(10)
items[-10:-9]
# 0
items[-20:-9]
# 0

正向指数从列表的开头开始计算。如果它们超调，它们也会在列表末尾固定：

items[9:10]
#[9]

items[9:100]
#[9]

与常规的 getitem 访问不同，列表边界之外的索引只是限制在最后。通常这会返回一个空切片：

items[99:100]
#[]

items[-99:-88]
#[]

你的例子中出现的特殊情况是因为'aeiou'[-1:0]与'aeiou'[4:0]是一回事 - 而且因为这是一个负长度但是正步幅的切片，所以它是空的。如果你输入'aeiou'[4:0:-1]，你将会回来'uoie'。

由于固定行为，无法保证返回的切片将包含与说明符之间距离相同的项目数。切片是一个很好的习惯用于获取没有事先检查的项目，这与我认为你要求的相反：

stuff = ['a','b','c']
d = stuff[11:12]
# []

stuff[11]
#IndexError: list index out of range

如果您确实需要知道您的查询超出范围，则必须预先检查索引：

def strict_slice(the_list, begin, end):
    listlen = len(the_list)
    if begin > listlen or begin + listlen < 0:
        raise IndexError, 'start index out of bounds'
    if end + listlen < 0 or end > listlen:
        raise IndexError, 'end index out of bounds'
    return the_list[begin:end]


test = 'abcdefg'
strict_slice(test, 1, 3)
strict_slice(test, -10, 3)
# IndexError: start index out of bounds
strict_slice(test, 1, 20)
#IndexError: end index out of bounds

虽然您可以继承list来实现这一点，但我似乎更容易使用此配方或只是检查空切片。

Answer 2

有时元组＆amp;列表包装问题（通过＆＃34; [ - 1：0]和＃34;边界）可以用＆＃34; MixIn＆＃34;修补。类。

e.g。的代码：

#!/usr/bin/env python def wrapslice(*sl): if len(sl)==1: sl=sl[0] if isinstance(sl, int): return sl else: if not isinstance(sl,slice): sl=slice(*sl) if None in (sl.start,sl.stop): return sl elif sl.start>=0 or sl.stop<0: return sl else: return slice(sl.start, None, sl.step) class WrapMixin(object): # Not sure how to use a decorator on a MixIn, so I use some "boilerplate" code... def __getitem__(self,sl): return super(WrapMixin,self).__getitem__(wrapslice(sl)) def __setitem__(self,sl, value): return super(WrapMixin,self).__setitem__(wrapslice(sl), value) def __delitem__(self,sl): return super(WrapMixin,self).__delitem__(wrapslice(sl)) # Note: for __getslice__ [-1:2] converts to [-1+len:2]... # i.e. add len to any -ve numbers! Ouch... def __getslice__(self,start,stop): if start>stop: stop+=len(self) sl=wrapslice(start,stop) return super(WrapMixin,self).__getslice__(sl.start, sl.stop) def __setslice__(self,start,stop, value): if start>stop: stop+=len(self) sl=wrapslice(start,stop) return super(WrapMixin,self).__setslice__(sl.start, sl.stop, value) def __delslice__(self,start,stop): if start>stop: stop+=len(self) sl=wrapslice(start,stop) return super(WrapMixin,self).__delslice__(sl.start, sl.stop) # Todo: def append/insert/append/iadd/radd ... #class WrapString(WrapMixin, string): pass # FAIL class WrapTuple(WrapMixin, tuple): pass # GOOD class WrapList(WrapMixin, list): pass # GOOD if __name__=="__main__": # test case aeiou=list("aeiou") AEIOU=WrapList("AEIOU") len_slice=3 for i in range(-5,5,1): vanilla=slice(i,i+len_slice) fixed=wrapslice(vanilla) print AEIOU[i]+"...","Vanilla:",vanilla,"vs Patched:",fixed print " get_slice VANILLA: %15r -> PATCHED: %15r"%(aeiou[vanilla.start:vanilla.stop], AEIOU[vanilla.start:vanilla.stop]) print " get_item VANILLA: %15r -> PATCHED: %15r"%(aeiou[vanilla], AEIOU[vanilla])

<强>输出：

A... Vanilla: slice(-5, -2, None) vs Patched: slice(-5, -2, None) get_slice VANILLA: ['a', 'e', 'i'] -> PATCHED: ['A', 'E', 'I'] get_item VANILLA: ['a', 'e', 'i'] -> PATCHED: ['A', 'E', 'I'] E... Vanilla: slice(-4, -1, None) vs Patched: slice(-4, -1, None) get_slice VANILLA: ['e', 'i', 'o'] -> PATCHED: ['E', 'I', 'O'] get_item VANILLA: ['e', 'i', 'o'] -> PATCHED: ['E', 'I', 'O'] I... Vanilla: slice(-3, 0, None) vs Patched: slice(-3, None, None) get_slice VANILLA: [] -> PATCHED: ['I', 'O', 'U'] get_item VANILLA: [] -> PATCHED: ['I', 'O', 'U'] O... Vanilla: slice(-2, 1, None) vs Patched: slice(-2, None, None) get_slice VANILLA: [] -> PATCHED: ['O', 'U'] get_item VANILLA: [] -> PATCHED: ['O', 'U'] U... Vanilla: slice(-1, 2, None) vs Patched: slice(-1, None, None) get_slice VANILLA: [] -> PATCHED: ['U'] <= Found U! get_item VANILLA: [] -> PATCHED: ['U'] A... Vanilla: slice(0, 3, None) vs Patched: slice(0, 3, None) get_slice VANILLA: ['a', 'e', 'i'] -> PATCHED: ['A', 'E', 'I'] get_item VANILLA: ['a', 'e', 'i'] -> PATCHED: ['A', 'E', 'I'] E... Vanilla: slice(1, 4, None) vs Patched: slice(1, 4, None) get_slice VANILLA: ['e', 'i', 'o'] -> PATCHED: ['E', 'I', 'O'] get_item VANILLA: ['e', 'i', 'o'] -> PATCHED: ['E', 'I', 'O'] I... Vanilla: slice(2, 5, None) vs Patched: slice(2, 5, None) get_slice VANILLA: ['i', 'o', 'u'] -> PATCHED: ['I', 'O', 'U'] get_item VANILLA: ['i', 'o', 'u'] -> PATCHED: ['I', 'O', 'U'] O... Vanilla: slice(3, 6, None) vs Patched: slice(3, 6, None) get_slice VANILLA: ['o', 'u'] -> PATCHED: ['O', 'U'] get_item VANILLA: ['o', 'u'] -> PATCHED: ['O', 'U'] U... Vanilla: slice(4, 7, None) vs Patched: slice(4, 7, None) get_slice VANILLA: ['u'] -> PATCHED: ['U'] get_item VANILLA: ['u'] -> PATCHED: ['U']

注意：字符串类型不能被子类化，因此无法修补另外：致电会员__getslice__＆amp;带有负面开头的__setslice__：stop args很棘手，因为这些参数会被＆＃39;摆弄。由翻译...因此额外的补丁代码......

通常只切片列表/元组的一个元素？

2 个答案: