diff -r a13310c045b0 Lib/test/test_socket.py
--- a/Lib/test/test_socket.py	Mon May 02 13:36:18 2011 +0200
+++ b/Lib/test/test_socket.py	Tue May 03 12:41:31 2011 +0300
@@ -274,6 +274,45 @@
         self.assertRaises(socket.error, raise_gaierror,
                               "Error raising socket exception.")
 
+    def testSendtoErrors(self):
+        # Testing that sendto doens't masks failures. See #10169.
+        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        self.addCleanup(s.close)
+        s.bind(('', 0))
+        sockname = s.getsockname()
+        # 2 args
+        with self.assertRaises(UnicodeEncodeError):
+            s.sendto(u'\u2620', sockname)
+        with self.assertRaises(TypeError) as cm:
+            s.sendto(5j, sockname)
+        self.assertIn('not complex', str(cm.exception))
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo', None)
+        self.assertIn('not NoneType', str(cm.exception))
+        # 3 args
+        with self.assertRaises(UnicodeEncodeError):
+            s.sendto(u'\u2620', 0, sockname)
+        with self.assertRaises(TypeError) as cm:
+            s.sendto(5j, 0, sockname)
+        self.assertIn('not complex', str(cm.exception))
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo', 0, None)
+        self.assertIn('not NoneType', str(cm.exception))
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo', 'bar', sockname)
+        self.assertIn('an integer is required', str(cm.exception))
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo', None, None)
+        self.assertIn('an integer is required', str(cm.exception))
+        # wrong number of args
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo')
+        self.assertIn('(1 given)', str(cm.exception))
+        with self.assertRaises(TypeError) as cm:
+            s.sendto('foo', 0, sockname, 4)
+        self.assertIn('(4 given)', str(cm.exception))
+
+
     def testCrucialConstants(self):
         # Testing for mission critical constants
         socket.AF_INET
diff -r a13310c045b0 Modules/socketmodule.c
--- a/Modules/socketmodule.c	Mon May 02 13:36:18 2011 +0200
+++ b/Modules/socketmodule.c	Tue May 03 12:41:31 2011 +0300
@@ -2821,18 +2821,25 @@
 sock_sendto(PySocketSockObject *s, PyObject *args)
 {
     Py_buffer pbuf;
-    PyObject *addro;
+    PyObject *addro, *tmp = NULL;
     char *buf;
     Py_ssize_t len;
     sock_addr_t addrbuf;
     int addrlen, n = -1, flags, timeout;
 
     flags = 0;
-    if (!PyArg_ParseTuple(args, "s*O:sendto", &pbuf, &addro)) {
-        PyErr_Clear();
-        if (!PyArg_ParseTuple(args, "s*iO:sendto",
-                              &pbuf, &flags, &addro))
+    /* sendto accepts an optional second arg 'flags' */
+    if (!PyArg_ParseTuple(args, "s*O|O:sendto", &pbuf, &addro, &tmp))
+        return NULL;
+    /* if tmp is set, 3 arguments have been passed, so convert the second
+       (flags) to int, and copy the third one to addro */
+    if (tmp != NULL) {
+        flags = PyInt_AsLong(addro);
+        if ((flags == -1) && PyErr_Occurred()) {
+            PyBuffer_Release(&pbuf);
             return NULL;
+        }
+        addro = tmp;
     }
     buf = pbuf.buf;
     len = pbuf.len;