diff -r 11dea186d3fc Modules/_io/_iomodule.h
--- a/Modules/_io/_iomodule.h	Wed Oct 13 15:15:02 2010 +0200
+++ b/Modules/_io/_iomodule.h	Wed Oct 13 22:59:32 2010 +0200
@@ -19,7 +19,6 @@ extern PyTypeObject PyBufferedRandom_Typ
 extern PyTypeObject PyTextIOWrapper_Type;
 extern PyTypeObject PyIncrementalNewlineDecoder_Type;
 
-
 extern int _PyIO_ConvertSsize_t(PyObject *, void *);
 
 /* These functions are used as METH_NOARGS methods, are normally called
@@ -41,6 +40,13 @@ extern int _PyIOBase_finalize(PyObject *
    Doesn't check the argument type, so be careful! */
 extern int _PyFileIO_closed(PyObject *self);
 
+/* Warn if the given RawIO object is not yet closed (used in dealloc
+   routines, hence the name). No-op if the object isn't a FileIO. */
+extern void _PyFileIO_dealloc_warn(PyObject *self, PyObject *source);
+
+/* Same with a BufferedIO object. */
+extern void _PyBufferedIO_dealloc_warn(PyObject *self, PyObject *source);
+
 /* Shortcut to the core of the IncrementalNewlineDecoder.decode method */
 extern PyObject *_PyIncrementalNewlineDecoder_decode(
     PyObject *self, PyObject *input, int final);
diff -r 11dea186d3fc Modules/_io/bufferedio.c
--- a/Modules/_io/bufferedio.c	Wed Oct 13 15:15:02 2010 +0200
+++ b/Modules/_io/bufferedio.c	Wed Oct 13 22:59:32 2010 +0200
@@ -197,6 +197,7 @@ typedef struct {
     int detached;
     int readable;
     int writable;
+    int deallocating;
     
     /* True if this is a vanilla Buffered object (rather than a user derived
        class) *and* the raw stream is a vanilla FileIO object. */
@@ -342,6 +343,7 @@ typedef struct {
 static void
 buffered_dealloc(buffered *self)
 {
+    self->deallocating = 1;
     if (self->ok && _PyIOBase_finalize((PyObject *) self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
@@ -435,6 +437,9 @@ buffered_close(buffered *self, PyObject 
         Py_INCREF(res);
         goto end;
     }
+
+    if (self->deallocating)
+        _PyBufferedIO_dealloc_warn((PyObject *) self, (PyObject *) self);
     /* flush() will most probably re-take the lock, so drop it first */
     LEAVE_BUFFERED(self)
     res = PyObject_CallMethodObjArgs((PyObject *)self, _PyIO_str_flush, NULL);
@@ -2297,3 +2302,16 @@ PyTypeObject PyBufferedRandom_Type = {
     PyType_GenericNew,          /* tp_new */
 };
 
+
+void
+_PyBufferedIO_dealloc_warn(PyObject *obj, PyObject *source)
+{
+    if (PyObject_TypeCheck(obj, &PyBufferedReader_Type) ||
+        PyObject_TypeCheck(obj, &PyBufferedWriter_Type) ||
+        PyObject_TypeCheck(obj, &PyBufferedRandom_Type)) {
+        buffered *self = (buffered *) obj;
+        if (self->raw) {
+            _PyFileIO_dealloc_warn(self->raw, source);
+        }
+    }
+}
diff -r 11dea186d3fc Modules/_io/fileio.c
--- a/Modules/_io/fileio.c	Wed Oct 13 15:15:02 2010 +0200
+++ b/Modules/_io/fileio.c	Wed Oct 13 22:59:32 2010 +0200
@@ -2,6 +2,7 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
+#include "structmember.h"
 #ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>
 #endif
@@ -55,6 +56,7 @@ typedef struct {
     unsigned int writable : 1;
     signed int seekable : 2; /* -1 means unknown */
     unsigned int closefd : 1;
+    unsigned int deallocating: 1;
     PyObject *weakreflist;
     PyObject *dict;
 } fileio;
@@ -69,6 +71,24 @@ _PyFileIO_closed(PyObject *self)
     return ((fileio *)self)->fd < 0;
 }
 
+void
+_PyFileIO_dealloc_warn(PyObject *obj, PyObject *source)
+{
+    fileio *self;
+
+    if (!PyFileIO_Check(obj))
+        return;
+    self = (fileio *) obj;
+    if (self->fd >= 0 && self->closefd) {
+        PyObject *exc, *val, *tb;
+        PyErr_Fetch(&exc, &val, &tb);
+        PyErr_SetString(PyExc_RuntimeWarning,
+                        "file not yet closed when deallocating");
+        PyErr_WriteUnraisable(source);
+        PyErr_Restore(exc, val, tb);
+    }
+}
+
 static PyObject *
 portable_lseek(int fd, PyObject *posobj, int whence);
 
@@ -110,6 +130,8 @@ fileio_close(fileio *self)
         self->fd = -1;
         Py_RETURN_NONE;
     }
+    if (self->deallocating)
+        _PyFileIO_dealloc_warn((PyObject *) self, (PyObject *) self);
     errno = internal_close(self);
     if (errno < 0)
         return NULL;
@@ -398,6 +420,7 @@ fileio_clear(fileio *self)
 static void
 fileio_dealloc(fileio *self)
 {
+    self->deallocating = 1;
     if (_PyIOBase_finalize((PyObject *) self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
diff -r 11dea186d3fc Modules/_io/textio.c
--- a/Modules/_io/textio.c	Wed Oct 13 15:15:02 2010 +0200
+++ b/Modules/_io/textio.c	Wed Oct 13 22:59:32 2010 +0200
@@ -658,6 +658,7 @@ typedef struct
     char writetranslate;
     char seekable;
     char telling;
+    char deallocating;
     /* Specialized encoding func (see below) */
     encodefunc_t encodefunc;
     /* Whether or not it's the start of the stream */
@@ -1094,6 +1095,7 @@ _textiowrapper_clear(textio *self)
 static void
 textiowrapper_dealloc(textio *self)
 {
+    self->deallocating = 1;
     if (_textiowrapper_clear(self) < 0)
         return;
     _PyObject_GC_UNTRACK(self);
@@ -2410,6 +2412,8 @@ textiowrapper_close(textio *self, PyObje
         Py_RETURN_NONE; /* stream already closed */
     }
     else {
+        if (self->deallocating)
+            _PyBufferedIO_dealloc_warn(self->buffer, (PyObject *) self);
         res = PyObject_CallMethod((PyObject *)self, "flush", NULL);
         if (res == NULL) {
             return NULL;