blocks|key|1236614|text|在对对象进行切片时，__getitem__()方法将接收slice对象。只需查看slice对象的start、stop和step成员，即可获得切片的组件。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1236615|>>>+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')|code-block|syntax|javascript|1236616|entityMap^0|A|D|S|5|14|5|1C|5|1I|4|1N|4|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|L|$]]

The <code>__getitem__()</code> method will receive a <code>slice</code> object when the object is sliced. Simply look at the <code>start</code>, <code>stop</code>, and <code>step</code> members of the <code>slice</code> object in order to get the components for the slice.

<pre><code>&gt;&gt;&gt; class C(object):
... def __getitem__(self, val):
... print val
... 
&gt;&gt;&gt; c = C()
&gt;&gt;&gt; c[3]
3
&gt;&gt;&gt; c[3:4]
slice(3, 4, None)
&gt;&gt;&gt; c[3:4:-2]
slice(3, 4, -2)
&gt;&gt;&gt; c[():1j:'a']
slice((), 1j, 'a')
</code></pre>

blocks|key|1236650|text|正确的方法是让__getitem__接受一个参数，这个参数可以是一个数字，也可以是一个切片对象。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1236651|请参见：|1236652|http://docs.python.org/library/functions.html#slice|1236653|http://docs.python.org/reference/datamodel.html#object.__getitem__|1236654|entityMap|0|LINK|mutability|MUTABLE|url|1^0|7|B|0|0|0|1F|0|0|0|1U|1|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|X|8|@]|D|@$9|Y|A|Z|1|10]]|E|$]]|$1|J|3|K|5|6|7|11|8|@]|D|@$9|12|A|13|1|14]]|E|$]]|$1|L|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|M|$N|$5|O|P|Q|E|$R|I]]|S|$5|O|P|Q|E|$R|K]]]]

The correct way to do this is to have <code>__getitem__</code> take one parameter, which can either be a number, or a slice object.

See:

<a href="http://docs.python.org/library/functions.html#slice" rel="nofollow noreferrer"><a href="http://docs.python.org/library/functions.html#slice" rel="nofollow noreferrer">http://docs.python.org/library/functions.html#slice</a></a>

<a href="http://docs.python.org/reference/datamodel.html#object.__getitem__" rel="nofollow noreferrer"><a href="http://docs.python.org/reference/datamodel.html#object.__getitem__" rel="nofollow noreferrer">http://docs.python.org/reference/datamodel.html#object.__getitem__</a></a>

blocks|key|1236707|text|我有一个“合成”列表(其中的数据比你想要在内存中创建的要大)，我的__getitem__看起来像这样：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1236708|def+__getitem__(+self,+key+)+:
++++if+isinstance(+key,+slice+)+:
++++++++#Get+the+start,+stop,+and+step+from+the+slice
++++++++return+[self[ii]+for+ii+in+xrange(*key.indices(len(self)))]
++++elif+isinstance(+key,+int+)+:
++++++++if+key+<+0+:+#Handle+negative+indices
++++++++++++key+%2B=+len(+self+)
++++++++if+key+<+0+or+key+>=+len(+self+)+:
++++++++++++raise+IndexError,+"The+index+(%25d)+is+out+of+range."%25key
++++++++return+self.getData(key)+#Get+the+data+from+elsewhere
++++else:
++++++++raise+TypeError,+"Invalid+argument+type."|code-block|syntax|javascript|1236709|切片不会返回相同的类型，这是一个禁忌，但它对我来说是有效的。|1236710|entityMap^0|X|B|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

I have a "synthetic" list (one where the data is larger than you would want to create in memory) and my <code>__getitem__</code> looks like this:

<pre><code>def __getitem__( self, key ) :
 if isinstance( key, slice ) :
 #Get the start, stop, and step from the slice
 return [self[ii] for ii in xrange(*key.indices(len(self)))]
 elif isinstance( key, int ) :
 if key &lt; 0 : #Handle negative indices
 key += len( self )
 if key &lt; 0 or key &gt;= len( self ) :
 raise IndexError, "The index (%d) is out of range."%key
 return self.getData(key) #Get the data from elsewhere
 else:
 raise TypeError, "Invalid argument type."
</code></pre>

The slice doesn't return the same type, which is a no-no, but it works for me.

blocks|key|1022068|text|为了扩展Aaron的答案，对于像numpy这样的东西，您可以通过检查given是否是tuple来执行多维切片|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1022069|class+Sliceable(object):
++++def+__getitem__(self,+given):
++++++++if+isinstance(given,+slice):
++++++++++++#+do+your+handling+for+a+slice+object:
++++++++++++print("slice",+given.start,+given.stop,+given.step)
++++++++elif+isinstance(given,+tuple):
++++++++++++print("multidim",+given)
++++++++else:
++++++++++++#+Do+your+handling+for+a+plain+index
++++++++++++print("plain",+given)

sliceme+=+Sliceable()
sliceme[1]
sliceme[::]
sliceme[1:,+::2]|code-block|syntax|javascript|1022070|Output:

```javascript|1022071|(‘普通’，1)|1022072|('slice'，无)|1022073|('multidim'，(slice(1，None，None)，slice(None，None，2)|1022074|1022075|entityMap^0|G|5|Y|5|16|5|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|M|3|N|5|6|7|14|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|15|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|16|8|@]|D|@]|E|$]]|$1|S|3|-4|5|H|7|17|8|@]|D|@]|E|$I|J]]|$1|T|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|U|$]]

To extend Aaron's answer, for things like <code>numpy</code>, you can do multi-dimensional slicing by checking to see if <code>given</code> is a <code>tuple</code>:

<pre><code>class Sliceable(object):
 def __getitem__(self, given):
 if isinstance(given, slice):
 # do your handling for a slice object:
 print("slice", given.start, given.stop, given.step)
 elif isinstance(given, tuple):
 print("multidim", given)
 else:
 # Do your handling for a plain index
 print("plain", given)

sliceme = Sliceable()
sliceme[1]
sliceme[::]
sliceme[1:, ::2]
</code></pre>

```

Output:

<pre><code>('plain', 1)
('slice', None, None, None)
('multidim', (slice(1, None, None), slice(None, None, 2)))
</code></pre>

I am trying to implement slice functionality for a class I am making that creates a vector representation.

I have this code so far, which I believe will properly implement the slice but whenever I do a call like <code>v[4]</code> where v is a vector python returns an error about not having enough parameters. So I am trying to figure out how to define the <code>getitem</code> special method in my class to handle both plain indexes and slicing.

<pre><code>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]
</code></pre>

Implementing slicing in __getitem__

我正在尝试为我创建的一个创建向量表示的类实现切片功能。到目前为止，我已经有了这段代码，我相信它可以正确地实现切片，但是每当我调用v[4] (其中v是一个向量python )时，都会返回一个关于没有足够参数的错误。因此，我正在尝试弄清楚如何在我的类中定义getitem特殊方法来处理普通索引和切片。def __getite...

问在getitem中实现切片
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