如何Python计算列表长度

Question

我想知道len()是如何工作的.

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

另外,我希望有人可以告诉我在哪里可以找到那些内置函数的源代码,如'len()','map()'等.

Answer 1

在这里下载Python 2.7源代码:http://www.python.org/getit/releases/2.7.4/

list在./Include/listobject.h和中实施./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__()只是咨询ob_size,这是一部分PyObject_VAR_HEAD.这使len()列表的常量操作成为可能.