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

Question

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'

Could you hint me what am I doing wrong? Thank you!

Asked By: HalfOfRain

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Source

Answer 1

I believe you have a typo in the code, specifically the _field_ part.
The problem is that it’s supposed to be _fields_ instead of _field_!

from ctypes import Structure

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

    class C(Structure):
        _fields_ = [("b", A.B)]

Hope this helps!

Another answer

You can move C out of A

from ctypes import Structure

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

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

this way, the C class can reference the A class because the A class has already been defined.

Answered By: Mark

Answer 2

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)]

Answered By: tdelaney

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

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