获取最近添加的项目的索引 [英] Get index of recently appended item

问题描述

是否有一种简单的方法来获取刚刚添加到列表中的项目的索引?我需要跟踪最后添加的项目.

Is there a straightforward way to get the index of an item I just appended to a list? I need to keep track of the last added item.

我想出了两种可能的解决方案:

I came up with two possible solutions:

# Workaround 1
# The last added is the one at index len(li) - 1
>> li = ['a', 'b', 'c',]
>> li.append('d')
>> last_index = len(li) - 1
>> last_item = li[len(li) - 1]

# Workaround 2
# Use of insert at index 0 so I know index of last added
>> li = ['a', 'b', 'c',]
>> li.insert(0, 'd')
>> last_item = li[0]

是否有技巧来获取附加项的索引?

Is there a trick to get the index of an appended item?

如果没有，您将使用上面的哪一个，为什么?您建议使用其他解决方法吗?

If there's not, which of the above would you use and why? Any different workaround you suggest?

推荐答案

li[-1]是列表中的最后一项，因此是最近添加到其末尾的项:

li[-1] is the last item in the list, and hence the one that was most recently appended to its end:

>>> li = [1, 2, 3]
>>> li.append(4)
>>> li[-1]
4

如果您需要索引而不是项目，那么len(li) - 1就很好并且非常有效(因为len(li)是在恒定时间内计算的-参见下文)

If you need the index, not the item, then len(li) - 1 is just fine, and very efficient (since len(li) is computed in constant time - see below)

在CPython的源代码中，列表的len映射到Objects/listobject.c中的函数list_length:

In the source of CPython, len for lists is mapped to function list_length in Objects/listobject.c:

static Py_ssize_t
list_length(PyListObject *a)
{
    return Py_SIZE(a);
}

Py_SIZE只是用于访问在Include/object.h中定义的所有Python对象的size属性的宏:

Py_SIZE is just a macro for accessing the size attribute of all Python objects, defined in Include/object.h:

#define Py_SIZE(ob)     (((PyVarObject*)(ob))->ob_size)

因此，len(lst)本质上是单个指针取消引用.

Hence, len(lst) is essentially a single pointer dereference.

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