diff -r bcc0f8eb6797 Lib/_pyio.py
--- a/Lib/_pyio.py	Sat Oct 10 02:09:41 2015 +0000
+++ b/Lib/_pyio.py	Sat Oct 10 16:52:18 2015 +0800
@@ -1390,6 +1390,7 @@
     _readable = False
     _writable = False
     _appending = False
+    _truncated = False
     _seekable = None
     _closefd = True
 
@@ -1440,6 +1441,7 @@
             flags = 0
         elif 'w' in mode:
             self._writable = True
+            self._truncated = True
             flags = os.O_CREAT | os.O_TRUNC
         elif 'a' in mode:
             self._writable = True
@@ -1716,7 +1718,10 @@
                 return 'ab'
         elif self._readable:
             if self._writable:
-                return 'rb+'
+                if self._truncated:
+                    return 'wb+'
+                else:
+                    return 'rb+'
             else:
                 return 'rb'
         else:
diff -r bcc0f8eb6797 Lib/test/test_fileio.py
--- a/Lib/test/test_fileio.py	Sat Oct 10 02:09:41 2015 +0000
+++ b/Lib/test/test_fileio.py	Sat Oct 10 16:52:18 2015 +0800
@@ -400,8 +400,8 @@
         # test that the mode attribute is correct for various mode strings
         # given as init args
         try:
-            for modes in [('w', 'wb'), ('wb', 'wb'), ('wb+', 'rb+'),
-                          ('w+b', 'rb+'), ('a', 'ab'), ('ab', 'ab'),
+            for modes in [('w', 'wb'), ('wb', 'wb'), ('wb+', 'wb+'),
+                          ('w+b', 'wb+'), ('a', 'ab'), ('ab', 'ab'),
                           ('ab+', 'ab+'), ('a+b', 'ab+'), ('r', 'rb'),
                           ('rb', 'rb'), ('rb+', 'rb+'), ('r+b', 'rb+')]:
                 # read modes are last so that TESTFN will exist first
diff -r bcc0f8eb6797 Lib/test/test_io.py
--- a/Lib/test/test_io.py	Sat Oct 10 02:09:41 2015 +0000
+++ b/Lib/test/test_io.py	Sat Oct 10 16:52:18 2015 +0800
@@ -3235,8 +3235,8 @@
 
         f = self.open(support.TESTFN, "w+")
         self.assertEqual(f.mode,            "w+")
-        self.assertEqual(f.buffer.mode,     "rb+") # Does it really matter?
-        self.assertEqual(f.buffer.raw.mode, "rb+")
+        self.assertEqual(f.buffer.mode,     "wb+")
+        self.assertEqual(f.buffer.raw.mode, "wb+")
 
         g = self.open(f.fileno(), "wb", closefd=False)
         self.assertEqual(g.mode,     "wb")
diff -r bcc0f8eb6797 Modules/_io/fileio.c
--- a/Modules/_io/fileio.c	Sat Oct 10 02:09:41 2015 +0000
+++ b/Modules/_io/fileio.c	Sat Oct 10 16:52:18 2015 +0800
@@ -63,6 +63,7 @@
     unsigned int readable : 1;
     unsigned int writable : 1;
     unsigned int appending : 1;
+    unsigned int truncated : 1;
     signed int seekable : 2; /* -1 means unknown */
     unsigned int closefd : 1;
     char finalizing;
@@ -193,6 +194,7 @@
         self->readable = 0;
         self->writable = 0;
         self->appending = 0;
+        self->truncated = 0;
         self->seekable = -1;
         self->blksize = 0;
         self->closefd = 1;
@@ -326,6 +328,7 @@
                 goto bad_mode;
             rwa = 1;
             self->writable = 1;
+            self->truncated = 1;
             flags |= O_CREAT | O_TRUNC;
             break;
         case 'a':
@@ -1042,7 +1045,10 @@
     }
     else if (self->readable) {
         if (self->writable)
-            return "rb+";
+            if (self->truncated)
+                return "wb+";
+            else
+                return "rb+";
         else
             return "rb";
     }