在 Python 中拦截切片操作答案

【问题标题】：Intercept slice operations in Python在 Python 中拦截切片操作
【发布时间】：2010-11-13 02:28:07
【问题描述】：

我想模仿一个普通的 python 列表，除了通过切片添加或删除元素时，我想“保存”列表。这可能吗？这是我的尝试，但它永远不会打印“保存”。

class InterceptedList(list):

    def addSave(func):
        def newfunc(self, *args):
            func(self, *args)
            print 'saving'
        return newfunc

    __setslice__ = addSave(list.__setslice__)
    __delslice__ = addSave(list.__delslice__)

>>> l = InterceptedList()
>>> l.extend([1,2,3,4])
>>> l
[1, 2, 3, 4]
>>> l[3:] = [5] # note: 'saving' is not printed
>>> l
[1, 2, 3, 5]

这确实适用于 append 和 extend 等其他方法，但不适用于切片操作。

编辑：真正的问题是我使用的是 Jython 而不是 Python，却忘了它。问题上的cmets是正确的。这段代码在 Python (2.6) 中运行良好。但是，代码和答案都可以在 Jython 中使用。

【问题讨论】：

我只是将您的代码复制并粘贴到 python 2.5.1 中，它按预期打印出“保存”。
它在这里工作。 Windows 上的 Python 2.6.2。
使用 2.5.2。在 extend() 操作期间打印“保存”，但在切片分配期间不打印。
可能是因为 setslice 已被弃用 tinyurl.com/kr3gxn
您使用的是什么版本的 jython？

标签： python methods jython slice intercept

【解决方案1】：

来自Python 3 docs：

__getslice__(), __setslice__() and __delslice__() were killed. 
The syntax a[i:j] now translates to a.__getitem__(slice(i, j)) 
(or __setitem__() or __delitem__(), when used as an assignment 
or deletion target, respectively).

【讨论】：

【解决方案2】：

猜想就够了。让我们开始使用事实吧？据我所知，底线是您必须覆盖这两组方法。

如果您想实现撤消/重做，您可能应该尝试使用撤消堆栈和一组可以自己执行（）/撤消（）的操作。

代码

import profile
import sys

print sys.version

class InterceptedList(list):

    def addSave(func):
        def newfunc(self, *args):
            func(self, *args)
            print 'saving'
        return newfunc

    __setslice__ = addSave(list.__setslice__)
    __delslice__ = addSave(list.__delslice__)


class InterceptedList2(list):

    def __setitem__(self, key, value):
        print 'saving'
        list.__setitem__(self, key, value)

    def __delitem__(self, key):
        print 'saving'
        list.__delitem__(self, key)


print("------------Testing setslice------------------")
l = InterceptedList()
l.extend([1,2,3,4])
profile.run("l[3:] = [5]")
profile.run("l[2:6] = [12, 4]")
profile.run("l[-1:] = [42]")
profile.run("l[::2] = [6,6]")

print("-----------Testing setitem--------------------")
l2 = InterceptedList2()
l2.extend([1,2,3,4])
profile.run("l2[3:] = [5]")
profile.run("l2[2:6] = [12,4]")
profile.run("l2[-1:] = [42]")
profile.run("l2[::2] = [6,6]")

Jython 2.5

C:\Users\wuu-local.pyza\Desktop>c:\jython2.5.0\jython.bat intercept.py
2.5.0 (Release_2_5_0:6476, Jun 16 2009, 13:33:26)
[Java HotSpot(TM) Client VM (Sun Microsystems Inc.)]
------------Testing setslice------------------
saving
         3 function calls in 0.035 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.000    0.000    0.000    0.000 intercept.py:9(newfunc)
        1    0.034    0.034    0.035    0.035 profile:0(l[3:] = [5])
        0    0.000             0.000          profile:0(profiler)


saving
         3 function calls in 0.005 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.001    0.001 <string>:0(<module>)
        1    0.001    0.001    0.001    0.001 intercept.py:9(newfunc)
        1    0.004    0.004    0.005    0.005 profile:0(l[2:6] = [12, 4])
        0    0.000             0.000          profile:0(profiler)


saving
         3 function calls in 0.012 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.000    0.000    0.000    0.000 intercept.py:9(newfunc)
        1    0.012    0.012    0.012    0.012 profile:0(l[-1:] = [42])
        0    0.000             0.000          profile:0(profiler)


         2 function calls in 0.004 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.004    0.004    0.004    0.004 profile:0(l[::2] = [6,6])
        0    0.000             0.000          profile:0(profiler)


-----------Testing setitem--------------------
         2 function calls in 0.004 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.004    0.004    0.004    0.004 profile:0(l2[3:] = [5])
        0    0.000             0.000          profile:0(profiler)


         2 function calls in 0.006 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.006    0.006    0.006    0.006 profile:0(l2[2:6] = [12,4])
        0    0.000             0.000          profile:0(profiler)


         2 function calls in 0.004 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 <string>:0(<module>)
        1    0.004    0.004    0.004    0.004 profile:0(l2[-1:] = [42])
        0    0.000             0.000          profile:0(profiler)


saving
         3 function calls in 0.007 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.002    0.002 <string>:0(<module>)
        1    0.001    0.001    0.001    0.001 intercept.py:20(__setitem__)
        1    0.005    0.005    0.007    0.007 profile:0(l2[::2] = [6,6])
        0    0.000             0.000          profile:0(profiler)

Python 2.6.2

C:\Users\wuu-local.pyza\Desktop>python intercept.py
2.6 (r26:66721, Oct  2 2008, 11:35:03) [MSC v.1500 32 bit (Intel)]
------------Testing setslice------------------
saving
         4 function calls in 0.002 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.002    0.002    0.002    0.002 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 intercept.py:9(newfunc)
        1    0.000    0.000    0.002    0.002 profile:0(l[3:] = [5])
        0    0.000             0.000          profile:0(profiler)


saving
         4 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 intercept.py:9(newfunc)
        1    0.000    0.000    0.000    0.000 profile:0(l[2:6] = [12, 4])
        0    0.000             0.000          profile:0(profiler)


saving
         4 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 intercept.py:9(newfunc)
        1    0.000    0.000    0.000    0.000 profile:0(l[-1:] = [42])
        0    0.000             0.000          profile:0(profiler)


         3 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 profile:0(l[::2] = [6,6])
        0    0.000             0.000          profile:0(profiler)


-----------Testing setitem--------------------
         3 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 profile:0(l2[3:] = [5])
        0    0.000             0.000          profile:0(profiler)


         3 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 profile:0(l2[2:6] = [12,4])
        0    0.000             0.000          profile:0(profiler)


         3 function calls in 0.000 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.000    0.000 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 profile:0(l2[-1:] = [42])
        0    0.000             0.000          profile:0(profiler)


saving
         4 function calls in 0.003 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    0.000    0.000 :0(setprofile)
        1    0.000    0.000    0.003    0.003 <string>:1(<module>)
        1    0.002    0.002    0.002    0.002 intercept.py:20(__setitem__)
        1    0.000    0.000    0.003    0.003 profile:0(l2[::2] = [6,6])
        0    0.000             0.000          profile:0(profiler)

【讨论】：

【解决方案3】：

"setslice" 和 "delslice" 已被弃用，如果你想进行拦截，你需要使用传递给 "setitem" 和 "delitem" 的 python 切片对象。如果您想同时拦截切片和普通访问，此代码在 python 2.6.2 中完美运行。

class InterceptedList(list):

def addSave(func):
    def newfunc(self, *args):
        func(self, *args)
        print 'saving'
    return newfunc

def __setitem__(self, key, value):
    print 'saving'
    list.__setitem__(self, key, value)

def __delitem__(self, key):
    print 'saving'
    list.__delitem__(self, key)

【讨论】：

如果您从列表继承 __setslice__ 和 __getslice__ 仍然存在并且根本不调用 setitem/getitem 则不正确。

【解决方案4】：

调用__getslice__ 和__setslice__ 的情况非常狭窄。具体来说，只有在使用常规切片时才会发生切片，其中第一个和结束元素只被提及一次。对于任何其他切片语法或根本没有切片，调用__getitem__ 或__setitem__。

【讨论】：