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?

Try adding the line print(self.obj_a, self.obj_b) at the start of __init__ - it might give you a clue what's going wrong. And bear in mind that some of what you label as "working" may just be "not crashing in an obvious way". — DavidW
– DavidW, Commented Nov 16, 2019 at 12:11

Esato · Accepted Answer · 2019-11-16 22:48:37Z

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!

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