Index: Include/pyerrors.h
===================================================================
--- Include/pyerrors.h	(revision 69200)
+++ Include/pyerrors.h	(working copy)
@@ -323,6 +323,16 @@
 PyAPI_FUNC(int) PyOS_vsnprintf(char *str, size_t size, const char  *format, va_list va)
 			Py_GCC_ATTRIBUTE((format(printf, 3, 0)));
 
+#if defined(_MSC_VER) && defined(_DEBUG)
+PyAPI_FUNC(void) _Py_SetCRTErrorHandler(int enter);
+#define Py_BEGIN_CRT_ERROR_HANDLING _Py_SetCRTErrorHandler(1);
+#define Py_END_CRT_ERROR_HANDLING _Py_SetCRTErrorHandler(0);
+#else
+/* no-ops */
+#define Py_BEGIN_CRT_ERROR_HANDLING
+#define Py_END_CRT_ERROR_HANDLING
+#endif
+
 #ifdef __cplusplus
 }
 #endif
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 69200)
+++ Modules/posixmodule.c	(working copy)
@@ -581,9 +581,14 @@
 	fd = PyObject_AsFileDescriptor(fdobj);
 	if (fd < 0)
 		return NULL;
+	/* Wherever we accept integral file descriptors, we must use
+	 * weaker error checking in the windows C runtime
+	 */
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	res = (*func)(fd);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (res < 0)
 		return posix_error();
 	Py_INCREF(Py_None);
@@ -6188,9 +6193,11 @@
 	int fd, res;
 	if (!PyArg_ParseTuple(args, "i:close", &fd))
 		return NULL;
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	res = close(fd);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (res < 0)
 		return posix_error();
 	Py_INCREF(Py_None);
@@ -6208,10 +6215,12 @@
 	int fd_from, fd_to, i;
 	if (!PyArg_ParseTuple(args, "ii:closerange", &fd_from, &fd_to))
 		return NULL;
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	for (i = fd_from; i < fd_to; i++)
 		close(i);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	Py_RETURN_NONE;
 }
 
@@ -6226,9 +6235,11 @@
 	int fd;
 	if (!PyArg_ParseTuple(args, "i:dup", &fd))
 		return NULL;
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	fd = dup(fd);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (fd < 0)
 		return posix_error();
 	return PyInt_FromLong((long)fd);
@@ -6245,9 +6256,11 @@
 	int fd, fd2, res;
 	if (!PyArg_ParseTuple(args, "ii:dup2", &fd, &fd2))
 		return NULL;
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	res = dup2(fd, fd2);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (res < 0)
 		return posix_error();
 	Py_INCREF(Py_None);
@@ -6289,6 +6302,7 @@
 	if (PyErr_Occurred())
 		return NULL;
 
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 #if defined(MS_WIN64) || defined(MS_WINDOWS)
 	res = _lseeki64(fd, pos, how);
@@ -6296,6 +6310,7 @@
 	res = lseek(fd, pos, how);
 #endif
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (res < 0)
 		return posix_error();
 
@@ -6325,9 +6340,11 @@
 	buffer = PyString_FromStringAndSize((char *)NULL, size);
 	if (buffer == NULL)
 		return NULL;
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	n = read(fd, PyString_AsString(buffer), size);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (n < 0) {
 		Py_DECREF(buffer);
 		return posix_error();
@@ -6351,9 +6368,12 @@
 
 	if (!PyArg_ParseTuple(args, "is*:write", &fd, &pbuf))
 		return NULL;
+	/* turn off crt asserts on windows since we have no control over fd */
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	size = write(fd, pbuf.buf, (size_t)pbuf.len);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 		PyBuffer_Release(&pbuf);
 	if (size < 0)
 		return posix_error();
@@ -6377,9 +6397,11 @@
         /* on OpenVMS we must ensure that all bytes are written to the file */
         fsync(fd);
 #endif
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 	res = FSTAT(fd, &st);
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	if (res != 0) {
 #ifdef MS_WINDOWS
 		return win32_error("fstat", NULL);
@@ -6419,6 +6441,7 @@
 		PyMem_FREE(mode);
 		return NULL;
 	}
+	Py_BEGIN_CRT_ERROR_HANDLING
 	Py_BEGIN_ALLOW_THREADS
 #if !defined(MS_WINDOWS) && defined(HAVE_FCNTL_H)
 	if (mode[0] == 'a') {
@@ -6438,6 +6461,7 @@
 	fp = fdopen(fd, mode);
 #endif
 	Py_END_ALLOW_THREADS
+	Py_END_CRT_ERROR_HANDLING
 	PyMem_FREE(mode);
 	if (fp == NULL)
 		return posix_error();
@@ -6455,10 +6479,13 @@
 static PyObject *
 posix_isatty(PyObject *self, PyObject *args)
 {
-	int fd;
+	int fd, atty;
 	if (!PyArg_ParseTuple(args, "i:isatty", &fd))
 		return NULL;
-	return PyBool_FromLong(isatty(fd));
+	Py_BEGIN_CRT_ERROR_HANDLING
+	atty = isatty(fd);
+	Py_END_CRT_ERROR_HANDLING
+	return PyBool_FromLong(atty);
 }
 
 #ifdef HAVE_PIPE
Index: Objects/exceptions.c
===================================================================
--- Objects/exceptions.c	(revision 69200)
+++ Objects/exceptions.c	(working copy)
@@ -1994,7 +1994,29 @@
 }
 #endif
 
+#if defined(_MSC_VER) && defined(_DEBUG)
+#include <crtdbg.h>
 
+static _CRT_REPORT_HOOK prevCrtReportHook;
+__declspec(thread) static int crt_report_hook_flag = 0; /* FALSE */
+
+/* A function for temporarily disabling strict CRT error handling on
+ * windows.
+ * this is needed where we we have to accept a "raw" parameter from the
+ * user and have the CRT respond leniently, e.g. by issuing a EINVAL.
+ * Examples are functions that take integral file descriptors,
+ * strftime which takes an arbitrary format string, and the
+ * mode string for open()
+ */
+void _Py_SetCRTErrorHandler(int enter) { crt_report_hook_flag = enter; }
+
+int __cdecl
+CrtReportHook(int reportType, char *message, int *returnValue)
+{
+    return crt_report_hook_flag;
+}
+#endif
+
 PyMODINIT_FUNC
 _PyExc_Init(void)
 {
@@ -2157,9 +2179,11 @@
 #if defined _MSC_VER && _MSC_VER >= 1400 && defined(__STDC_SECURE_LIB__)
     /* Set CRT argument error handler */
     prevCrtHandler = _set_invalid_parameter_handler(InvalidParameterHandler);
-    /* turn off assertions in debug mode */
-    prevCrtReportMode = _CrtSetReportMode(_CRT_ASSERT, 0);
 #endif
+#if defined(_MSC_VER) && defined(_DEBUG)
+    /* turn off assertions between Py_(BEGIN|END)_CRT_ERROR_HANDLING */
+    prevCrtReportHook = _CrtSetReportHook(CrtReportHook);
+#endif
 }
 
 void
@@ -2170,6 +2194,8 @@
 #if defined _MSC_VER && _MSC_VER >= 1400 && defined(__STDC_SECURE_LIB__)
     /* reset CRT error handling */
     _set_invalid_parameter_handler(prevCrtHandler);
-    _CrtSetReportMode(_CRT_ASSERT, prevCrtReportMode);
 #endif
+#if defined(_MSC_VER) && defined(_DEBUG)
+    _CrtSetReportHook(prevCrtReportHook);
+#endif
 }