Index: src/org/python/core/PyTableCode.java
===================================================================
--- src/org/python/core/PyTableCode.java	(revision 7109)
+++ src/org/python/core/PyTableCode.java	(working copy)
@@ -99,23 +99,22 @@
     }
 
     public PyObject __findattr_ex__(String name) {
-        // have to craft co_varnames specially
-        if (name == "co_varnames") {
+        if (name.equals("co_varnames")) {
             return toPyStringTuple(co_varnames);
         }
-        if (name == "co_cellvars") {
+        if (name.equals("co_cellvars")) {
             return toPyStringTuple(co_cellvars);
         }
-        if (name == "co_freevars") {
+        if (name.equals("co_freevars")) {
             return toPyStringTuple(co_freevars);
         }
-        if (name == "co_filename") {
+        if (name.equals("co_filename")) {
             return new PyString(co_filename);
         }
-        if (name == "co_name") {
+        if (name.equals("co_name")) {
             return new PyString(co_name);
         }
-        if (name == "co_flags") {
+        if (name.equals("co_flags")) {
             return Py.newInteger(co_flags.toBits());
         }
         return super.__findattr_ex__(name);
Index: src/org/python/core/PyString.java
===================================================================
--- src/org/python/core/PyString.java	(revision 7109)
+++ src/org/python/core/PyString.java	(working copy)
@@ -2,6 +2,8 @@
 package org.python.core;
 
 import java.math.BigInteger;
+import java.util.Map;
+import java.util.Set;
 
 import org.python.core.util.ExtraMath;
 import org.python.core.util.StringUtil;
@@ -9,6 +11,8 @@
 import org.python.expose.ExposedNew;
 import org.python.expose.ExposedType;
 import org.python.expose.MethodType;
+import org.python.util.Generic;
+import com.google.common.collect.MapMaker;
 
 /**
  * A builtin python string.
@@ -17,8 +21,12 @@
 public class PyString extends PyBaseString
 {
     public static final PyType TYPE = PyType.fromClass(PyString.class);
-    protected String string; // cannot make final because of Python intern support
-    protected transient boolean interned=false;
+    private static final Set<String> internedStrings;
+    static {
+        Map<String, Boolean> tempMap = new MapMaker().weakKeys().makeMap();
+        internedStrings = Generic.newSetFromMap(tempMap);
+    }
+    protected final String string;
 
     public String getString() {
         return string;
@@ -56,7 +64,7 @@
      */
     public static PyString fromInterned(String interned) {
         PyString str = new PyString(TYPE, interned);
-        str.interned = true;
+        str.intern();
         return str;
     }
 
@@ -78,6 +86,10 @@
         }
     }
 
+    protected void intern() {
+        internedStrings.add(getString());
+    }
+
     public int[] toCodePoints() {
         int n = getString().length();
         int[] codePoints = new int[n];
@@ -125,13 +137,8 @@
     }
 
     public String internedString() {
-        if (interned)
-            return getString();
-        else {
-            string = getString().intern();
-            interned = true;
-            return getString();
-        }
+        intern();
+        return getString();
     }
 
     @Override
Index: src/org/python/core/PyUnicode.java
===================================================================
--- src/org/python/core/PyUnicode.java	(revision 7109)
+++ src/org/python/core/PyUnicode.java	(working copy)
@@ -114,7 +114,7 @@
      */
     public static PyUnicode fromInterned(String interned) {
         PyUnicode uni = new PyUnicode(TYPE, interned);
-        uni.interned = true;
+        uni.intern();
         return uni;
     }