Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 67680)
+++ Lib/io.py	(working copy)
@@ -914,6 +914,8 @@
         mode. If n is negative, read until EOF or until read() would
         block.
         """
+        if self.closed:
+            raise ValueError("I/O operation on closed file.")
         with self._read_lock:
             return self._read_unlocked(n)