As a result of a regression that snuck into 2.5.3 add a test case that

ensures that when you try to read from a file opened for writing an IOError
is raised.

Lib/test/test_file.py

@@ -120,6 +120,8 @@ class AutoFileTests(unittest.TestCase):
         except:
             self.assertEquals(self.f.__exit__(*sys.exc_info()), None)
 
+    def testReadWhenWriting(self):
+        self.assertRaises(IOError, self.f.read)
 
 class OtherFileTests(unittest.TestCase):
 

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Added test case to ensure attempts to read from a file opened for writing
+  fail.
+
 - Issue #2467: gc.DEBUG_STATS reported invalid elapsed times. Also, always
   print elapsed times, not only when some objects are uncollectable /
   unreachable. Original patch by Neil Schemenauer.