Python：在__getitem__中实现切片

Question

我正在尝试为要创建的类创建矢量表示形式的切片功能。

到目前为止，我已经有了这段代码，我相信它将正确实现切片，但是每当我进行诸如v[4]v是矢量的调用时，python都会返回有关参数不足的错误。因此，我试图找出如何getitem在我的类中定义特殊方法来处理纯索引和切片。

def __getitem__(self, start, stop, step):

    index = start

    if stop == None:

        end = start + 1

    else:

        end = stop

    if step == None:

        stride = 1

    else:

        stride = step

    return self.__data[index:end:stride]

Answer 1

切片对象时，该__getitem__()方法将接收一个slice对象。简单地看start，stop和step对成员的slice对象，以获得该片段的组件。>>> class C(object):...   def __getitem__(self, val):...     print val... >>> c = C()>>> c[3]3>>> c[3:4]slice(3, 4, None)>>> c[3:4:-2]slice(3, 4, -2)>>> c[():1j:'a']slice((), 1j, 'a')

Answer 2

我有一个“合成”列表（数据大于您要在内存中创建的列表），我的__getitem__样子是这样的：def __getitem__( self, key ) :&nbsp; &nbsp; if isinstance( key, slice ) :&nbsp; &nbsp; &nbsp; &nbsp; #Get the start, stop, and step from the slice&nbsp; &nbsp; &nbsp; &nbsp; return [self[ii] for ii in xrange(*key.indices(len(self)))]&nbsp; &nbsp; elif isinstance( key, int ) :&nbsp; &nbsp; &nbsp; &nbsp; if key < 0 : #Handle negative indices&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; key += len( self )&nbsp; &nbsp; &nbsp; &nbsp; if key < 0 or key >= len( self ) :&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; raise IndexError, "The index (%d) is out of range."%key&nbsp; &nbsp; &nbsp; &nbsp; return self.getData(key) #Get the data from elsewhere&nbsp; &nbsp; else:&nbsp; &nbsp; &nbsp; &nbsp; raise TypeError, "Invalid argument type."切片不会返回相同的类型，这是不可以的，但是对我有用。

Answer 3

如何定义getitem类以处理纯索引和切片？切片对象当您使用的下标符号冒号被自动创建的-而这正是传递给__getitem__。使用isinstance来检查，如果你有一个切片对象：from __future__ import print_functionclass Sliceable(object):&nbsp; &nbsp; def __getitem__(self, subscript):&nbsp; &nbsp; &nbsp; &nbsp; if isinstance(subscript, slice):&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; # do your handling for a slice object:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; print(subscript.start, subscript.stop, subscript.step)&nbsp; &nbsp; &nbsp; &nbsp; else:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; # Do your handling for a plain index&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; print(subscript)假设我们使用的是范围对象，但我们希望切片返回列表，而不是新的范围对象（确实如此）：>>> range(1,100, 4)[::-1]range(97, -3, -4)由于内部限制，我们不能对范围进行子类化，但是我们可以委托给它：class Range:&nbsp; &nbsp; """like builtin range, but when sliced gives a list"""&nbsp; &nbsp; __slots__ = "_range"&nbsp; &nbsp; def __init__(self, *args):&nbsp; &nbsp; &nbsp; &nbsp; self._range = range(*args) # takes no keyword arguments.&nbsp; &nbsp; def __getattr__(self, name):&nbsp; &nbsp; &nbsp; &nbsp; return getattr(self._range, name)&nbsp; &nbsp; def __getitem__(self, subscript):&nbsp; &nbsp; &nbsp; &nbsp; result = self._range.__getitem__(subscript)&nbsp; &nbsp; &nbsp; &nbsp; if isinstance(subscript, slice):&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return list(result)&nbsp; &nbsp; &nbsp; &nbsp; else:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return resultr = Range(100)我们没有完美可替换的Range对象，但它非常接近：>>> r[1:3][1, 2]>>> r[1]1>>> 2 in rTrue>>> r.count(3)1为了更好地理解切片符号，这是Sliceable的示例用法：>>> sliceme = Sliceable()>>> sliceme[1]1>>> sliceme[2]2>>> sliceme[:]None None None>>> sliceme[1:]1 None None>>> sliceme[1:2]1 2 None>>> sliceme[1:2:3]1 2 3>>> sliceme[:2:3]None 2 3>>> sliceme[::3]None None 3>>> sliceme[::]None None None>>> sliceme[:]None None NonePython 2，请注意：在Python 2中，有一个不赞成使用的方法，在子类化某些内置类型时可能需要重写该方法。从数据模型文档中：object.__getslice__(self, i, j)从2.0版开始不推荐使用：支持将切片对象用作__getitem__()方法的参数。（但是，CPython中的内置类型当前仍在实现__getslice__()。因此，在实现切片时必须在派生类中重写它。）这在Python 3中已经消失了。