list 对象，容量自适应的数组式容器

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# 新建一个列表
>>> l = [1, 2, 3]
>>> l
[1, 2, 3]

# 向尾部追加元素，列表对象视情况自动扩容
>>> l.append(4)
>>> l
[1, 2, 3, 4]

# 从尾部弹出元素，列表对象视情况自动缩容
>>> l.pop()
4
>>> l
[1, 2, 3]

# 向头部插入元素，该操作需要挪动后面的元素，谨慎使用！
>>> l.insert(0, 4)
>>> l
[4, 1, 2, 3]

# 从头部弹出元素，该操作需要挪动后面的元素，谨慎使用！
>>> l.pop(0)
4
>>> l
[1, 2, 3]

# 查找元素第一次出现位置的下标
>>> l.index(2)
1

# 用一个可迭代对象扩展列表——元素逐一追加到尾部
>>> l.extend([1, 2])
>>> l
[1, 2, 3, 1, 2]

# 计算元素出现的个数
>>> l.count(1)
2
>>> l.count(3)
1

# 将列表反转
>>> l.reverse()
>>> l
[2, 1, 3, 2, 1]

# 将列表清空
>>> l.clear()
>>> l
[]

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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;