diff -r ece75a3b942c Lib/cgitb.py
--- a/Lib/cgitb.py	Wed Dec 05 17:59:29 2012 +0200
+++ b/Lib/cgitb.py	Thu Dec 06 22:14:38 2012 +0100
@@ -85,7 +85,10 @@
         if ttype == tokenize.NAME and token not in keyword.kwlist:
             if lasttoken == '.':
                 if parent is not __UNDEF__:
-                    value = getattr(parent, token, __UNDEF__)
+                    try:
+                        value = getattr(parent, token, __UNDEF__)
+                    except Exception:
+                        value = __UNDEF__
                     vars.append((prefix + token, prefix, value))
             else:
                 where, value = lookup(token, frame, locals)
diff -r ece75a3b942c Lib/test/test_cgitb.py
--- a/Lib/test/test_cgitb.py	Wed Dec 05 17:59:29 2012 +0200
+++ b/Lib/test/test_cgitb.py	Thu Dec 06 22:14:38 2012 +0100
@@ -62,6 +62,19 @@
         self.assertNotIn('<p>', out)
         self.assertNotIn('</p>', out)
 
+    def test_exception_in_gettattr_call(self):
+        # Issue 4643: cgitb.html fails if getattr call raises exception
+        class WeirdObject(object):
+            def __getattr__(self, attr):
+                if attr == 'a':
+                    return str(slf) # Intentional NameError
+                raise AttributeError(attr)
+        try:
+            weird = WeirdObject()
+            weird.a
+        except Exception:
+            html_bt = cgitb.html(sys.exc_info())
+
 
 def test_main():
     run_unittest(TestCgitb)