如何计算列表长度 [英] How to Python count list length

问题描述

我想知道 len() 是如何工作的.

I want to know how len() works.

每次我调用 len() 时它是否从列表的开头到结尾计数，或者，由于 list 也是一个类，len() 是否只返回列表对象中记录列表长度的变量?

Does it count from beginning to end of a list every time I call len(), or, since list is also an class, does len() just return a variable in the list object which record the list's length?

另外，我希望有人能告诉我在哪里可以找到那些内置函数的源代码，比如'len()'、'map()'等

Also, I hope someone can tell me where I can find the source code of those built-in functions like 'len()', 'map()', etc.

推荐答案

在此处下载 Python 2.7 源代码:http://www.python.org/getit/releases/2.7.4/

Download Python 2.7 source code here: http://www.python.org/getit/releases/2.7.4/

list 在 ./Include/listobject.h 和 ./Objects/listobject.c 中实现.

list is implemented in ./Include/listobject.h and ./Objects/listobject.c.

typedef struct {
    PyObject_VAR_HEAD
    /* Vector of pointers to list elements.  list[0] is ob_item[0], etc. */
    PyObject **ob_item;

    /* ob_item contains space for 'allocated' elements.  The number
     * currently in use is ob_size.
     * Invariants:
     *     0 <= ob_size <= allocated
     *     len(list) == ob_size
     *     ob_item == NULL implies ob_size == allocated == 0
     * list.sort() temporarily sets allocated to -1 to detect mutations.
     *
     * Items must normally not be NULL, except during construction when
     * the list is not yet visible outside the function that builds it.
     */
    Py_ssize_t allocated;
} PyListObject;

list.__len__() 只是参考了 ob_size，它是 PyObject_VAR_HEAD 的一部分.这使得 len() 成为列表的恒定时间操作.

list.__len__() just consults ob_size, which is part of PyObject_VAR_HEAD. This makes len() a constant-time operation for lists.

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