why PyObject_VAR_HEAD?

[kio]

Hi,

I'm studying the CPython source code. I don’t quite understand why
they’re using PyObject_VAR_HEAD to define struct like PyListObject. To
define such kind of struct, could I use _PyObject_HEAD_EXTRA as a
header and add "items" pointer and "allocated" count explicity? Is
there any difference?

Thanks.

Eric