Load dll with Python Ctypes

Make sure your compiler and Python version are both 32-bit or 64-bit. You can't mix and that's what causes it OSError: [WinError 193] %1 is not a valid Win32 application.

Next, make sure to compile as a C program and not C++. That's why the name mangling is mentioned in your answer.

Example (note that the compiler is for x86 not x64 :

C:\>cl /LD /W4 test.c
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.61030 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

test.c
Microsoft (R) Incremental Linker Version 11.00.61030.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.dll
/dll
/implib:test.lib
test.obj
   Creating library test.lib and object test.exp

Now using 32-bit Python:

C:\>py -2
Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> lib = CDLL('test')
>>> lib.sum(2,3)
5

If you compile in C++, you can still call functions by exporting them as C, which prevents C++ name clutter:

//test.cpp
extern "C" __declspec(dllexport) int sum(int a, int b) {
    return a + b;
}