Python supports having a C extension for the the __init__ of a package (instead of having __init__.py). This works fine on Linux, but on Windows distutils fails to build the C extension because it assumes the entry point is named PyInit___init__ while importlib expects PyInit_*package* (for a package named *package*). 

When building the extension I get the following error:

LINK : error LNK2001: unresolved external symbol PyInit___init__
build\temp.win32-3.7\Release\__init__.cp37-win32.lib : fatal error LNK1120: 1 unresolved externals


The code below can be used to reproduce the issue.

Setup.py (extracted from a larger setup.py, but should work...):

from setuptools import setup, Extension
extension3 = Extension("ext_package.__init__", sources=["init.c"])

setup(
    ext_modules=[extension3],
)

Source code for the module (init.c):

#include "Python.h"
  

static PyModuleDef mod_def = {
        PyModuleDef_HEAD_INIT,
        "ext_package.__init__",
        NULL,
        0,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL
};

PyObject* PyInit_ext_package(void)
{
        return PyModule_Create(&mod_def);
}


P.S. I cannot easily debug this, I ran into this when testing one of my projects on AppVeyor and don't have a local Windows machine.