Python源码解析之List

lists = []
list.append('name')
list.append('age')
list.append('grade')

typedef struct{
	struct _object *_ob_next;
	struct _object *_ob_prev; 	// python内部将对象放在链表进行内存管理
	Py_ssize_t ob_refcnt;		// 引用计数器，就是多少变量用了它
	PyObject **ob_item;			// 指针的指针，存列表的元素
	Py_ssize_t ob_size;			// 已有元素个数
	Py_ssize_t allocated;		// 列表容量，可容纳个数
} PyListObject;

PyObject *
PyList_New(Py_ssize_t size)
{
    PyListObject *op;
    size_t nbytes;
#ifdef SHOW_ALLOC_COUNT
    static int initialized = 0;
    if (!initialized) {
        Py_AtExit(show_alloc);
        initialized = 1;
    }
#endif
    // 缓存机制
    if (size < 0) {
        PyErr_BadInternalCall();
        return NULL;
    }
    /* Check for overflow without an actual overflow,
     *  which can cause compiler to optimise out */
    if ((size_t)size > PY_SIZE_MAX / sizeof(PyObject *))
        return PyErr_NoMemory();
    nbytes = size * sizeof(PyObject *);
    if (numfree) {
        numfree--;
        op = free_list[numfree];
        _Py_NewReference((PyObject *)op);
#ifdef SHOW_ALLOC_COUNT
        count_reuse++;
#endif
    } else {
        op = PyObject_GC_New(PyListObject, &PyList_Type);
        if (op == NULL)
            return NULL;Py
#ifdef SHOW_ALLOC_COUNT
        count_alloc++;
#endif
    }

    if (size <= 0)
        op->ob_item = NULL;
    else {
        op->ob_item = (PyObject **) PyMem_MALLOC(nbytes);
        if (op->ob_item == NULL) {
            Py_DECREF(op);
            return PyErr_NoMemory();
        }
        memset(op->ob_item, 0, nbytes);
    }
    Py_SIZE(op) = size;  // 元素个数
    op->allocated = size;   // 容量
    _PyObject_GC_TRACK(op); //放到双向链表进行维护
    return (PyObject *) op; //返回列表的指针
}

// 添加元素
static int
app1(PyListObject *self, PyObject *v)
{
    // 获取实际元素个数
    Py_ssize_t n = PyList_GET_SIZE(self);

    assert (v != NULL);
    if (n == PY_SSIZE_T_MAX) {
        PyErr_SetString(PyExc_OverflowError,
            "cannot add more objects to list");
        return -1;
    }

    // 计算当前容量和内部元素个数
    // 直接添加元素/扩容添加
    if (list_resize(self, n+1) == -1)
        return -1;
    // 将元素添加到ob_item，v
    Py_INCREF(v);
    PyList_SET_ITEM(self, n, v);
    return 0;
}

// 扩容机制
 // newsize: 已存在元素个数+1
static int
list_resize(PyListObject *self, Py_ssize_t newsize)
{
    PyObject **items;
    size_t new_allocated;
    Py_ssize_t allocated = self->allocated; // 当前的容量

    // 1,容量大于个数
    // 2,个数大于容量的一半（容量足够且没有内存浪费）
    if (allocated >= newsize && newsize >= (allocated >> 1)) {
        assert(self->ob_item != NULL || newsize == 0);
        Py_SIZE(self) = newsize;
        return 0;
    }

    /* 
     * The growth pattern is:  0, 4, 8, 16, 25, 35, 46, 58, 72, 88, ...
     */
     // 扩容机制的算法
    new_allocated = (newsize >> 3) + (newsize < 9 ? 3 : 6);

    /* check for integer overflow */
    if (new_allocated > PY_SIZE_MAX - newsize) {
        PyErr_NoMemory();
        return -1;
    } else {
        new_allocated += newsize;
    }

    if (newsize == 0)
        new_allocated = 0;
    // 扩容/缩容（涉及原来元素的迁移）
    items = self->ob_item;
    if (new_allocated <= (PY_SIZE_MAX / sizeof(PyObject *)))
        PyMem_RESIZE(items, PyObject *, new_allocated);
    else
        items = NULL;
    if (items == NULL) {
        PyErr_NoMemory();
        return -1;
    }
    // 赋值，更新个数和容量
    self->ob_item = items;
    Py_SIZE(self) = newsize;
    self->allocated = new_allocated;
    return 0;
}

static PyObject *
listpop(PyListObject *self, PyObject *args)
{
    Py_ssize_t i = -1;
    PyObject *v;
    int status;

    if (!PyArg_ParseTuple(args, "|n:pop", &i))
        return NULL;

    if (Py_SIZE(self) == 0) {
        /* Special-case most common failure cause */
        PyErr_SetString(PyExc_IndexError, "pop from empty list");
        return NULL;
    }
    if (i < 0)
        i += Py_SIZE(self);
    if (i < 0 || i >= Py_SIZE(self)) {
        PyErr_SetString(PyExc_IndexError, "pop index out of range");
        return NULL;
    }
    v = self->ob_item[i];
    // 删除最后一个，仅改变size
    if (i == Py_SIZE(self) - 1) {
        status = list_resize(self, Py_SIZE(self) - 1);
        assert(status >= 0);
        return v; /* and v now owns the reference the list had */
    }
    Py_INCREF(v);
    // 不是最后一个，需要移动数据位置
    status = list_ass_slice(self, i, i+1, (PyObject *)NULL);
    assert(status >= 0);
    /* Use status, so that in a release build compilers don't
     * complain about the unused name.
     */
    (void) status;

    return v;
}

static int
list_clear(PyListObject *a)
{
    Py_ssize_t i;
    PyObject **item = a->ob_item;
    if (item != NULL) {
        i = Py_SIZE(a);
        // 各个元素设置为空
        Py_SIZE(a) = 0;
        a->ob_item = NULL;
        a->allocated = 0;
        // 引用计数器-1
        while (--i >= 0) {
            Py_XDECREF(item[i]);
        }
        PyMem_FREE(item);
    }
 
    return 0;
}

static void
list_dealloc(PyListObject *op)
{
    Py_ssize_t i;
    // 判断引用计数是否为0
    PyObject_GC_UnTrack(op);
    Py_TRASHCAN_SAFE_BEGIN(op)
    if (op->ob_item != NULL) {
        i = Py_SIZE(op);
        while (--i >= 0) {
            Py_XDECREF(op->ob_item[i]);
        }
        PyMem_FREE(op->ob_item);
    }
    // free_list没有80个的话缓存这个list
    if (numfree < PyList_MAXFREELIST && PyList_CheckExact(op))
        free_list[numfree++] = op;
    else
        Py_TYPE(op)->tp_free((PyObject *)op);
    Py_TRASHCAN_SAFE_END(op)
}

# 两次list的地址相同
>>> list1=[1,2,3]
>>> id(list1)
69070216L
>>> del list1
>>> list2=[0,0,0]
>>> id(list2)
69303304L
>>>