Negative array indicies and slice()

[Ian Kelly]

On Mon, Oct 29, 2012 at 7:49 PM, Chris Kaynor <ckaynor at zindagigames.com> wrote:
> NOTE: The above is taken from reading the source code for Python 2.6.
> For some odd reason, I am getting that an empty tuple consists of 6
> pointer-sized objects (48 bytes on x64), rather than the expected 3
> pointer-sized (24 bytes on x64). Slices are showing up as the expected
> 5 pointer-sized (40 bytes on x64), and tuples grow at the expected 1
> pointer (8 bytes on x64) per item. I imagine I am missing something,
> but cannot figure out what that would be.

I'm likewise seeing 4 extra words in tuples in 32-bit Python 3.3.
What I've found is that for tuples and other collection objects, the
garbage collector tacks on an extra header before the object in
memory.  That header looks like this:

typedef union _gc_head {
    struct {
        union _gc_head *gc_next;
        union _gc_head *gc_prev;
        Py_ssize_t gc_refs;
    } gc;
    long double dummy;  /* force worst-case alignment */
} PyGC_Head;

gc_next and gc_prev implement a doubly-linked list that the garbage
collector uses to explicitly track this object.  gc_refs is used for
counting references during a garbage collection and stores the GC
state of the object otherwise.

I'm not entirely certain why collection objects get this special
treatment, but there you have it.