Index: Lib/test/test_memoryio.py
===================================================================
--- Lib/test/test_memoryio.py	(revision 73313)
+++ Lib/test/test_memoryio.py	(working copy)
@@ -9,6 +9,7 @@
 import io
 import _pyio as pyio
 import sys
+        
 
 class MemorySeekTestMixin:
 
@@ -338,7 +339,14 @@
         self.assertEqual(test1(), buf)
         self.assertEqual(test2(), buf)
 
+    def test_instance_dict_leak(self):
+        # Test case for issue #6242.
+        # This will be caught by regrtest.py -R if this leak.
+        for _ in range(100):
+            memio = self.ioclass()
+            memio.foo = 1
 
+
 class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
 
     UnsupportedOperation = pyio.UnsupportedOperation
Index: Modules/_io/bytesio.c
===================================================================
--- Modules/_io/bytesio.c	(revision 73313)
+++ Modules/_io/bytesio.c	(working copy)
@@ -606,14 +606,31 @@
     Py_RETURN_NONE;
 }
 
+static int
+bytesio_traverse(BytesIOObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->dict);
+    return 0;
+}
+
+static int
+bytesio_clear(BytesIOObject *self)
+{
+    Py_CLEAR(self->dict);
+    return 0;
+}
+
 static void
 bytesio_dealloc(BytesIOObject *self)
 {
-    if (self->buf != NULL) {
+    _PyObject_GC_UNTRACK(self);
+    if (self->buf) {
         PyMem_Free(self->buf);
         self->buf = NULL;
     }
-    Py_TYPE(self)->tp_clear((PyObject *)self);
+    Py_CLEAR(self->dict);
+    if (self->weakreflist != NULL)
+        PyObject_ClearWeakRefs((PyObject *)self);
     Py_TYPE(self)->tp_free(self);
 }
 
@@ -663,24 +680,6 @@
     return 0;
 }
 
-static int
-bytesio_traverse(BytesIOObject *self, visitproc visit, void *arg)
-{
-    Py_VISIT(self->dict);
-    Py_VISIT(self->weakreflist);
-    return 0;
-}
-
-static int
-bytesio_clear(BytesIOObject *self)
-{
-    Py_CLEAR(self->dict);
-    if (self->weakreflist != NULL)
-        PyObject_ClearWeakRefs((PyObject *)self);
-    return 0;
-}
-
-
 static PyGetSetDef bytesio_getsetlist[] = {
     {"closed",  (getter)bytesio_get_closed, NULL,
      "True if the file is closed."},
Index: Modules/_io/stringio.c
===================================================================
--- Modules/_io/stringio.c	(revision 73313)
+++ Modules/_io/stringio.c	(working copy)
@@ -509,11 +509,15 @@
 stringio_dealloc(StringIOObject *self)
 {
     _PyObject_GC_UNTRACK(self);
+    self->ok = 0;
+    if (self->buf) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
     Py_CLEAR(self->readnl);
     Py_CLEAR(self->writenl);
     Py_CLEAR(self->decoder);
-    if (self->buf)
-        PyMem_Free(self->buf);
+    Py_CLEAR(self->dict);
     if (self->weakreflist != NULL)
         PyObject_ClearWeakRefs((PyObject *) self);
     Py_TYPE(self)->tp_free(self);