Export PyInit_pkg for pkg.__init__ instead of PyInit___init__

Author: abravalheri

distutils/command/build_ext.py

@@ -729,7 +729,7 @@ def get_export_symbols(self, ext):
         provided, "PyInit_" + module_name.  Only relevant on Windows, where
         the .pyd file (DLL) must export the module "PyInit_" function.
         """
-        name = ext.name.split('.')[-1]
+        name = self._get_module_name_for_symbol(ext)
         try:
             # Unicode module name support as defined in PEP-489
             # https://peps.python.org/pep-0489/#export-hook-name
@@ -744,6 +744,15 @@ def get_export_symbols(self, ext):
             ext.export_symbols.append(initfunc_name)
         return ext.export_symbols
 
+    def _get_module_name_for_symbol(self, ext):
+        # Package name should be used for `__init__` modules
+        # https://github.com/python/cpython/issues/80074
+        # https://github.com/pypa/setuptools/issues/4826
+        parts = ext.name.split(".")
+        if parts[-1] == "__init__" and len(parts) >= 2:
+            return parts[-2]
+        return parts[-1]
+
     def get_libraries(self, ext):  # noqa: C901
         """Return the list of libraries to link against when building a
         shared extension.  On most platforms, this is just 'ext.libraries';

distutils/tests/test_build_ext.py

@@ -345,6 +345,19 @@ def test_unicode_module_names(self):
         assert cmd.get_export_symbols(modules[0]) == ['PyInit_foo']
         assert cmd.get_export_symbols(modules[1]) == ['PyInitU_f_1gaa']
 
+    def test_export_symbols__init__(self):
+        # https://github.com/python/cpython/issues/80074
+        # https://github.com/pypa/setuptools/issues/4826
+        modules = [
+            Extension('foo.__init__', ['aaa']),
+            Extension('föö.__init__', ['uuu']),
+        ]
+        dist = Distribution({'name': 'xx', 'ext_modules': modules})
+        cmd = self.build_ext(dist)
+        cmd.ensure_finalized()
+        assert cmd.get_export_symbols(modules[0]) == ['PyInit_foo']
+        assert cmd.get_export_symbols(modules[1]) == ['PyInitU_f_1gaa']
+
     def test_compiler_option(self):
         # cmd.compiler is an option and
         # should not be overridden by a compiler instance