Why Python's list structure store the PyObject using **ob_item instead of *ob_item

Question

Python's list structure is like below:

typedef struct {
    PyObject_VAR_HEAD
    PyObject **ob_item;
    Py_ssize_t allocated;
} PyListObject;

Why using **ob_item? I think *ob_item is enough.

Are you under the impression that the list's storage physically contains its elements, like a C++ vector or something? The list just holds pointers to its elements. — user2357112
– user2357112, Commented Jan 8, 2017 at 3:44

jeffknupp · Accepted Answer · 2017-01-08 03:59:04Z

2

Because, as with any list created using pointers, as you add and remove elements, you'll need to change what a node's pointer is pointing to, not just the value of the object being pointed to (which is what you would get with just *ob_item; **ob_item let's you "repoint" the pointer).

Remember, the list is not going to contain the actual objects, it contains pointers to them. To be able to use a pointer that points to a pointer (to, as I mentioned, be able to repoint the pointer for a node), you need **.

edited Jan 8, 2017 at 3:59

answered Jan 8, 2017 at 3:39

jeffknupp

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