Here is a short explanation. For a longer explanation, read A New C API for CPython (September 2017) by Victor Stinner.
Given the following code:
typedef struct {
PyVarObject ob_base;
PyObject **ob_item; // <-- pointer to the array of list items
Py_ssize_t allocated;
} PyListObject;
#define PyList_GET_ITEM(op, i) ((PyListObject *)op)->ob_item[i]
And the following C code:
PyObject *item = PyList_GET_ITEM(list, 0);
On a 64-bit machine, the machine code of a release build becomes something like:
PyObject **items = (PyObject **)(((char*)op) + 24); PyObject *item = items[0];
whereas a debug build uses an offset of 40 instead of 24, because PyVarObject contains additional fields for debugging purpose:
PyObject **items = (PyObject **)(((char*)op) + 40); PyObject *item = items[0];
As a consequence, the compiled C extension is incompatible at the ABI level: a C extension has to be build twice, once in release mode and once in debug mode.
To reduce the maintenance burden, Linux vendors only provide C extensions compiled in release mode, making the debug mode mostly unusable on Linux in practice.
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