pythonctypes

Inner class cannot see the definition of another inner class with Python Ctypes


I tried converting C code snippet to Python by using Ctypes, but it doesn't work. The C code is as below:

struct A
{
    struct B
    {
        // Empty
    };

    struct C
    {
        B b;
    };
};

My Python code is:

from ctypes import Structure

class A(Structure):
    class B(Structure):
        _fields_ = []

    class C(Structure):
        _fields_ = [("b", A.B)]   # Error: Unresolved reference 'A'

What am I doing wrong?


Solution

  • In python, a class is bound to its name after the entire class has been defined. A does not exist for the duration of the definition. The ctypes Structures and unions shows how to construct nested structures. Each is defined as an outer class separately and its the Structure._fields_ attribute that makes one subordinate to the other.

    from ctypes import Structure
    
    class B(Structure):
        _fields_ = []
    
    class C(Structure):
        _fields_ = [("b", B)]
    
    class A(Structure):
        _fields_ = [("B", B), ("C", C)]