I'm new to cython and maybe i'm missing some base info, so be patient. What i want to do is create a c object in python, modify it and return the object's pointer to a c function. Basically i have:

// headers.h

class A {
    A();
    void modifyA()
}

class B {
    B();
    void useA(A *a);
}

# headers.pxd

cdef extern from "headers.h":
    cdef cppclass A:
        void modifyA()

    cdef cppclass B:
        void useA(A *a)

# PyA.pyx

cdef class PyA:
    cdef A *pa

    def __cinit__(self):
        self.pa = new A()

    def modifyA(self):
        self.pa.modifyA()

Now, as far as i understand, when i instantiate PyA from python code, an A object is created in c and a pointer is stored inside the new PyA object. What i want to do is use that pointer like:

# PyB.pyx

cdef class PyB:
    cdef B *b

    def __cinit__(self):
        self.b = new B()

    def useA(self, pyA: PyA):
        self.b.useA(pyA.pa)

but it gives me "Cannot convert Python object to 'A *'" and i can't understand why... Is there something i'm missing?

CodePudding user response：

The "Cannot convert Python object to 'Something'" error in Cython generally means Cython is not detecting the type of an object/property, and thus believes it will be a Python object, only available at runtime.

That being said, you have to make sure Cython understands the type. In your particular case, you can choose between:

• Having a single .pyx (merging both .pyx files), for cython to be able to directly determine that pyA.pa is of type A*.
• Declaring a "cython header file" or .pxd file, which will work much like a .h file in C by declaring types of objects, functions and classes.

The second case would look like this:

# PyA.pxd
cdef class PyA:
    cdef A *pa

# PyB.pyx
from PyA cimport PyA    # import the PyA class from PyA.pxd definition
cdef class PyB:
# [...]