Python 切片操作方法，我知道 Python 切片，但如何使用内置切片对象呢？
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Python slice how-to, I know the Python slice but how can I use built-in slice object for it?
提问by necromancer
What's the use of built-in function sliceand how can I use it?
The direct way of Pythonic slicing I know - l1[start:stop:step]. I want to know if I have a slice object, then how do I use it?
内置函数有什么用，slice如何使用？
我知道的 Pythonic 切片的直接方式 - l1[start:stop:step]. 我想知道我是否有切片对象，那么我如何使用它？
采纳答案by PaulMcG
You create a slice by calling slice with the same fields you would use if doing [start:end:step] notation:
如果使用 [start:end:step] 表示法，您可以通过调用具有相同字段的切片来创建切片：
sl = slice(0,4)
To use the slice, just pass it as if it were the index into a list or string:
要使用切片，只需将它作为索引传递给列表或字符串：
>>> s = "ABCDEFGHIJKL"
>>> sl = slice(0,4)
>>> print(s[sl])
'ABCD'
Let's say you have a file of fixed-length text fields. You could define a list of slices to easily extract the values from each "record" in this file.
假设您有一个固定长度文本字段的文件。您可以定义一个切片列表，以便轻松地从此文件中的每个“记录”中提取值。
data = """\
0010GEORGE JETSON 12345 SPACESHIP ST HOUSTON TX
0020WILE E COYOTE 312 ACME BLVD TUCSON AZ
0030FRED FLINTSTONE 246 GRANITE LANE BEDROCK CA
0040JONNY QUEST 31416 SCIENCE AVE PALO ALTO CA""".splitlines()
fieldslices = [slice(*fielddef) for fielddef in [
(0,4), (4, 21), (21,42), (42,56), (56,58),
]]
fields = "id name address city state".split()
for rec in data:
for field,sl in zip(fields, fieldslices):
print("{} : {}".format(field, rec[sl]))
print('')
Prints:
印刷：
id : 0010
name : GEORGE JETSON
address : 12345 SPACESHIP ST
city : HOUSTON
state : TX
id : 0020
name : WILE E COYOTE
address : 312 ACME BLVD
city : TUCSON
state : AZ
id : 0030
name : FRED FLINTSTONE
address : 246 GRANITE LANE
city : BEDROCK
state : CA
id : 0040
name : JONNY QUEST
address : 31416 SCIENCE AVE
city : PALO ALTO
state : CA
回答by Tim McNamara
The slicefunction returns slice objects. Slice objects are one of Python's internal types, which are optimized for read performance - all of their attributes are read-only.
该slice函数返回切片对象。Slice 对象是 Python 的内部类型之一，针对读取性能进行了优化——它们的所有属性都是只读的。
Altering slicecould be useful if wish to change the default behaviour. For example, lxmluses slice notation to access DOM elements (however, I haven't confirmed how they did that myself).
slice如果希望更改默认行为，更改可能很有用。例如，lxml使用切片表示法访问 DOM 元素（但是，我自己还没有确认他们是如何做到的）。
回答by Don O'Donnell
Square brackets following a sequence denote either indexing or slicing depending on what's inside the brackets:
序列后面的方括号表示索引或切片，具体取决于括号内的内容：
>>> "Python rocks"[1] # index
'y'
>>> "Python rocks"[1:10:2] # slice
'yhnrc'
Both of these cases are handled by the __getitem__()method of the sequence (or __setitem__()if on the left of an equals sign.) The index or slice is passed to the methods as a single argument, and the way Python does this is by converting the slice notation, (1:10:2, in this case) to a slice object: slice(1,10,2).
这两种情况都由__getitem__()序列的方法处理（或者__setitem__()如果在等号的左侧。）索引或切片作为单个参数传递给方法，Python 这样做的方式是通过转换切片表示法, ( 1:10:2, 在本例中) 到切片对象：slice(1,10,2).
So if you are defining your own sequence-like class or overriding the __getitem__or __setitem__or __delitem__methods of another class, you need to test the index argument to determine if it is an intor a slice, and process accordingly:
因此，如果您要定义自己的类似序列的类或覆盖另一个类的__getitem__or__setitem__或__delitem__方法，则需要测试 index 参数以确定它是 anint还是 a slice，并进行相应处理：
def __getitem__(self, index):
if isinstance(index, int):
... # process index as an integer
elif isinstance(index, slice):
start, stop, step = index.indices(len(self)) # index is a slice
... # process slice
else:
raise TypeError("index must be int or slice")
A sliceobject has three attributes: start, stopand step, and one method: indices, which takes a single argument, the length of the object, and returns a 3-tuple: (start, stop, step).
一个slice对象具有三个属性：start，stop和step，以及一个方法：indices，它接受一个参数，对象的长度，并返回一个三元组：(start, stop, step)。
回答by SingleNegationElimination
>>> class sl:
... def __getitem__(self, *keys): print keys
...
>>> s = sl()
>>> s[1:3:5]
(slice(1, 3, 5),)
>>> s[1:2:3, 1, 4:5]
((slice(1, 2, 3), 1, slice(4, 5, None)),)
>>>
回答by Roman Susi
While trying to answer Subset a string based on variable, I recalled that numpy has a syntactically nice way to define slice objects:
在尝试回答Subset a string based on variable 时，我回想起 numpy 有一种语法上很好的方式来定义切片对象：
>>> import numpy as np
>>> s = "The long-string instrument is a musical instrument in which the string is of such a length that the fundamental transverse wave is below what a person can hear as a tone."
>>> z = np.s_[18:26] # in this case same as slice(18, 26, None)
>>> s[z]
'strument'
The problem solved here is how to store the slice in a variable for later use, and np.s_allows to do just that. Yes, it's not built-in, but as that original question was redirected here I feel like my answer belong here as well. Also, numpy was one of the reasons why so advanced slicing abilities were added to Python, IIRC.
这里解决的问题是如何将切片存储在变量中供以后使用，并np.s_允许这样做。是的，它不是内置的，但是由于原始问题被重定向到这里，我觉得我的答案也属于这里。此外，numpy 是将如此先进的切片功能添加到 Python IIRC 的原因之一。
An example of a more complex "slicing":
一个更复杂的“切片”的例子：
>>> data = np.array(range(6)).reshape((2, 3))
>>> z = np.s_[:1, 1:2]
>>> data[z]
array([[1]])
>>> data
array([[0, 1, 2],
[3, 4, 5]])
>>> z
(slice(None, 1, None), slice(1, 2, None))
where z is now a tuple of slices.
其中 z 现在是切片元组。
