pythoncapi
diff --git a/‎Lib/test/test_io.py‎
Lines changed: 16 additions & 0 deletions b/‎Lib/test/test_io.py‎
Lines changed: 16 additions & 0 deletions
diff --git a/‎Misc/NEWS‎
Lines changed: 3 additions & 0 deletions b/‎Misc/NEWS‎
Lines changed: 3 additions & 0 deletions
diff --git a/‎Modules/_io/fileio.c‎
Lines changed: 7 additions & 1 deletion b/‎Modules/_io/fileio.c‎
Lines changed: 7 additions & 1 deletion
@@ -809,6 +809,22 @@ def opener(path, flags):
         with self.open("non-existent", "r", opener=opener) as f:
             self.assertEqual(f.read(), "egg\n")
 
+    def test_bad_opener_negative_1(self):
+        # Issue #27066.
+        def badopener(fname, flags):
+            return -1
+        with self.assertRaises(ValueError) as cm:
+            open('non-existent', 'r', opener=badopener)
+        self.assertEqual(str(cm.exception), 'opener returned -1')
+
+    def test_bad_opener_other_negative(self):
+        # Issue #27066.
+        def badopener(fname, flags):
+            return -2
+        with self.assertRaises(ValueError) as cm:
+            open('non-existent', 'r', opener=badopener)
+        self.assertEqual(str(cm.exception), 'opener returned -2')
+
     def test_fileio_closefd(self):
         # Issue #4841
         with self.open(__file__, 'rb') as f1, \
 
@@ -10,6 +10,9 @@ What's New in Python 3.6.0 alpha 2
 Core and Builtins
 -----------------
 
+- Issue #27066: Fixed SystemError if a custom opener (for open()) returns a
+  negative number without setting an exception.
+
 - Issue #26983: float() now always return an instance of exact float.
   The deprecation warning is emitted if __float__ returns an instance of
   a strict subclass of float.  In a future versions of Python this can
 
@@ -420,7 +420,13 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode,
 
             self->fd = _PyLong_AsInt(fdobj);
             Py_DECREF(fdobj);
-            if (self->fd == -1) {
+            if (self->fd < 0) {
+                if (!PyErr_Occurred()) {
+                    /* The opener returned a negative but didn't set an
+                       exception.  See issue #27066 */
+                    PyErr_Format(PyExc_ValueError,
+                                 "opener returned %d", self->fd);
+                }
                 goto error;
             }
         }
Original file line number	Diff line number	Diff line change
`@@ -420,7 +420,13 @@ _io_FileIO___init___impl(fileio self, PyObject nameobj, const char *mode,`
`420`	`420`
`421`	`421`	`self->fd = _PyLong_AsInt(fdobj);`
`422`	`422`	`Py_DECREF(fdobj);`
`423`		`- if (self->fd == -1) {`
	`423`	`+ if (self->fd < 0) {`
	`424`	`+ if (!PyErr_Occurred()) {`
	`425`	`+ /* The opener returned a negative but didn't set an`
	`426`	`+ exception. See issue #27066 */`
	`427`	`+ PyErr_Format(PyExc_ValueError,`
	`428`	`+ "opener returned %d", self->fd);`
	`429`	`+ }`
`424`	`430`	`goto error;`
`425`	`431`	`}`
`426`	`432`	`}`