How to clear a PyList object that contains objects?

Question

I am trying to clear a Python list I pass to a C function using ctypes. The calls I found at this link seem to work sometimes, but when the list I pass in contains other lists or classes as elements I get a segfault.

I have a file foo.py that looks like this:

import ctypes
_libpractice=ctypes.CDLL('./_practice.so', mode=ctypes.RTLD_GLOBAL)

_libpractice.update.argtypes = [ctypes.py_object]
_libpractice.update.restype = ctypes.py_object

def c_update(my_list):
    return _libpractice.update(my_list)

and a file practice.c that looks like this:

#include <Python.h>
#include <stdio.h>

PyObject* update(PyObject* list){
    Py_INCREF(list);
    Py_ssize_t len = PySequence_Length(list);    
    PySequence_DelSlice(list, 0, len);

    return list;
}

Here is an example of it working correctly with various primitive data types.

>>> import foo
>>> a = [1,2,3,4,5.5,'a',6]
>>> foo.c_update(a)
[]
>>>

Here is an example of the segfault:

>>> b = [1,2,3,[],5]
>>> foo.c_update(b)
Segmentation fault (core dumped)

I have figured out a way around this by calling PySequence_GetItem() on every item in the list and then calling PySequence_DelSlice() like so:

PyObject* update(PyObject* list){
    Py_INCREF(list);
    Py_ssize_t len = PySequence_Length(list);

    for(Py_ssize_t i = 0; i<len; i++){
        PyObject* item = PySequence_GetItem(list,i);
    }

    PySequence_DelSlice(list, 0, len);    

    return list;
} 

However, if I pass a class as an element of the list, the destructor is not called after it is deleted from the list.

>>> import foo
>>> class Sample:
...     def __del__(self):
...         print('del called')
...
>>> a = Sample()
>>> a = 3
del called
>>> b = Sample()
>>> my_list = [1,b,3,[]]
>>> foo.c_update(my_list)
[]
>>>

How can I clear the list I am passing in while at the same time making sure I am correctly handling reference counts?

Answer 1

I don't know if this is the direct cause of the segfault, but you cannot use the Python API from within a library loaded as CDLL. You have to use PyDLL. With CDLL, Python will release the GIL before calling functions from that library, and you need the GIL.