python
diff --git a/Lib/ctypes/__init__.py b/Lib/ctypes/__init__.py
@@ -481,7 +481,9 @@ def LoadLibrary(self, name):
 
     def WinError(code=None, descr=None):
         if code is None:
-            code = GetLastError()
+            # GetLastError returns a DWORD, cast large error codes
+            # to unsigned
+            code = GetLastError() & 0xffffffff
         if descr is None:
             descr = FormatError(code).strip()
         return OSError(None, descr, None, code)

diff --git a/Lib/test/test_ctypes/test_win32.py b/Lib/test/test_ctypes/test_win32.py
@@ -90,6 +90,26 @@ def test_winerror(self):
         self.assertEqual(e.errno, errno.EINVAL)
         self.assertEqual(e.winerror, ERROR_INVALID_PARAMETER)
 
+    def test_winerror_dword(self):
+        # see Issue 28474
+        E_POINTER = 0x80000005
+        msg = FormatError(E_POINTER).strip()
+        args = (E_POINTER, msg, None, E_POINTER)
+
+        e = WinError(E_POINTER)
+        self.assertEqual(e.args, args)
+        self.assertEqual(e.errno, E_POINTER)
+        self.assertEqual(e.winerror, E_POINTER)
+
+        windll.kernel32.SetLastError(E_POINTER)
+        try:
+            raise WinError()
+        except OSError as exc:
+            e = exc
+        self.assertEqual(e.args, args)
+        self.assertEqual(e.errno, E_POINTER)
+        self.assertEqual(e.winerror, E_POINTER)
+
 class Structures(unittest.TestCase):
     def test_struct_by_value(self):
         class POINT(Structure):

@@ -402,6 +402,14 @@ def test_WindowsError(self):
             self.assertEqual(w.strerror, 'foo')
             self.assertEqual(w.filename, None)
             self.assertEqual(w.filename2, None)
+            # DWORD error code (issue #28474)
+            E_POINTER = 0x80000005
+            w = OSError(E_POINTER, 'foo', 'bar', E_POINTER)
+            self.assertEqual(w.errno, E_POINTER)
+            self.assertEqual(w.winerror, E_POINTER)
+            self.assertEqual(w.strerror, 'foo')
+            self.assertEqual(w.filename, 'bar')
+            self.assertEqual(w.filename2, None)
 
     @unittest.skipUnless(sys.platform == 'win32',
                          'test specific to Windows')

diff --git a/Misc/NEWS.d/next/Windows/2021-08-26-00-58-35.bpo-27484.7MThNi.rst b/Misc/NEWS.d/next/Windows/2021-08-26-00-58-35.bpo-27484.7MThNi.rst
@@ -0,0 +1,2 @@
+Improve handling and reporting of unsigned win32 error codes.
+Patch by Adam Meily.
diff --git a/Modules/_ctypes/callproc.c b/Modules/_ctypes/callproc.c
@@ -1334,12 +1334,19 @@ static PyObject *format_error(PyObject *self, PyObject *args)
 {
     PyObject *result;
     wchar_t *lpMsgBuf;
-    DWORD code = 0;
-    if (!PyArg_ParseTuple(args, "|i:FormatError", &code))
+    long long code = 0;
+
+    if (!PyArg_ParseTuple(args, "|L:FormatError", &code))
+        return NULL;
+
+    if((DWORD)code != code) {
+        PyErr_Format(PyExc_OverflowError, "error code %lld too big for int", code);
         return NULL;
+    }
+
     if (code == 0)
         code = GetLastError();
-    lpMsgBuf = FormatError(code);
+    lpMsgBuf = FormatError((DWORD)code);
     if (lpMsgBuf) {
         result = PyUnicode_FromWideChar(lpMsgBuf, wcslen(lpMsgBuf));
         LocalFree(lpMsgBuf);

@@ -1700,15 +1700,21 @@ oserror_parse_args(PyObject **p_args,
             return -1;
 #ifdef MS_WINDOWS
         if (*winerror && PyLong_Check(*winerror)) {
-            long errcode, winerrcode;
+            long long errcode, winerrcode;
             PyObject *newargs;
             Py_ssize_t i;
 
-            winerrcode = PyLong_AsLong(*winerror);
+            winerrcode = PyLong_AsLongLong(*winerror);
             if (winerrcode == -1 && PyErr_Occurred())
                 return -1;
-            errcode = winerror_to_errno(winerrcode);
-            *myerrno = PyLong_FromLong(errcode);
+
+            if(winerrcode < LONG_MIN || winerrcode > ULONG_MAX) {
+                PyErr_Format(PyExc_OverflowError, "error code %lld too big for int", winerrcode);
+                return -1;
+            }
+
+            errcode = (long long)winerror_to_errno((int)winerrcode);
+            *myerrno = PyLong_FromLongLong(errcode);
             if (!*myerrno)
                 return -1;
             newargs = PyTuple_New(nargs);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Improve handling and reporting of unsigned win32 error codes.
		Patch by Adam Meily.