Strange behaviour when creating python attributes in cython cdef class

Question

We have given Cython code:

cdef extern from "C_File_A.h":
    cdef struct C_Obj_A:
        pass

cdef extern from "C_File_B.h":
    cdef struct C_Obj_B:
        pass

cdef class pC_Obj_A:
    cdef const C_Obj_A * _c_self

cdef class pC_Obj_B:
    cdef const C_Obj_B * _c_self

cdef class pC_Obj_C:
    cdef const C_Obj_A * _c_a
    cdef const C_Obj_B * _c_b


cdef class Obj_A_Wrap(pC_Obj_A):
    def __init__(self, pC_Obj_C obj_c):
        self._c_self = obj_c._c_a


cdef class Obj_B_Wrap(pC_Obj_B):
    def __init__(self, pC_Obj_C obj_c):
        self._c_self = obj_c._c_b


cdef class Stack:
    cdef public pC_Obj_A obj_a
    cdef public pC_Obj_B obj_b

    def __init__(self, pC_Obj_C obj_c):
        # Working
        self.obj_a = Obj_A_Wrap(obj_c)
        self.obj_b = Obj_B_Wrap(obj_c)

        # Working
        self.obj_a._c_self = obj_c._c_a
        self.obj_b = Obj_B_Wrap(obj_c)

        # Working
        self.obj_a = Obj_A_Wrap(obj_c)
        self.obj_b._c_self = obj_c._c_b

        # Not working
        self.obj_a._c_self = obj_c._c_a
        self.obj_b._c_self = obj_c._c_b

I need a python object Stack with attrubutes accessible from Python, so I have added to Stack class cdef public pC_Obj_A obj_a and cdef public pC_Obj_B obj_b.These objects are wrappers to the C struct pointers.

When I initialize these objects with intermediary wrappers i.e. Obj_A_Wrap everything is fine.

When I initialize one of these objects directly i.e. self.obj_a._c_self = obj_c._c_a also everything is fine.

When both obj_a and obj_b are initialized directly (# Not Working part of code) I have got strange behaviour of my C library that inlcude C_File_A and C_File_B and respectively the C structs definitions. The behaviour is similar to memory corruption, or overwriting some parts of the memory that should not be.

I have no idea why the direct initialization causes this strange behaviour. Maybe you know?

Answer 1

I have found the solution of my problem. When I was trying to solve this problem I have printed only _c_self attribute of the given object to check that the pointer was properly assigned and it was but when I printed entire object it turned out that python object is None instead of proper object declared as attribute.

print(self.obj_a, self.obj_b) # 66f000c0 66f000c0
print(f'{<int>self.obj_a._c_self:x} {<int>self.obj_b._c_self:x}') # None None

The solution is to add Create function to cdef class:

cdef class pC_Obj_A:
    cdef const C_Obj_A * _c_self

    @staticmethod
    cdef Create(C_Obj_A * ptr):
        cdef pC_Obj_A result = pC_Obj_A()
        result._c_self = ptr
        return result

And use it like this:

cdef class Stack:
    cdef public pC_Obj_A obj_a
    cdef public pC_Obj_B obj_b

    def __init__(self, pC_Obj_C obj_c):
        self.obj_a = pC_Obj_A.Create(obj_c._c_a)
        self.obj_b = pC_Obj_B.Create(obj_c._c_b)

Then printout is:

print(self.obj_a, self.obj_b) # <pC_Obj_A object at 0x029FF610> <pC_Obj_B object at 0x029FF620>
print(f'{<int>self.obj_a._c_self:x} {<int>self.obj_b._c_self:x}') # 2134b9c 2134c08

And everything works great!