Index: Modules/threadmodule.c
===================================================================
--- Modules/threadmodule.c	(revision 64565)
+++ Modules/threadmodule.c	(working copy)
@@ -293,7 +293,10 @@
 		}
 		
 	}
-	else if (self->dict != ldict) {
+
+	/* The call to tp_init above may have caused another thread to run.
+	   Install our ldict again. */
+	if (self->dict != ldict) {
 		Py_CLEAR(self->dict);
 		Py_INCREF(ldict);
 		self->dict = ldict;
Index: Lib/test/test_threading_local.py
===================================================================
--- Lib/test/test_threading_local.py	(revision 64565)
+++ Lib/test/test_threading_local.py	(working copy)
@@ -42,6 +42,32 @@
         deadlist = [weak for weak in weaklist if weak() is None]
         self.assert_(len(deadlist) in (n-1, n), (n, len(deadlist)))
 
+    def test_derived(self):
+        # Issue 3088: if there is a threads switch inside the __init__
+        # of a threading.local derived class, the per-thread dictionary
+        # is created but not correctly set on the object.
+        # The first member set may be bogus.
+        import time
+        class Local(threading.local):
+            def __init__(self):
+                time.sleep(0.01)
+        local = Local()
+
+        def f(i):
+            local.x = i
+            # Simply check that the variable is correctly set
+            self.assertEqual(local.x, i)
+
+        threads= []
+        for i in range(10):
+            t = threading.Thread(target=f, args=(i,))
+            t.start()
+            threads.append(t)
+
+        for t in threads:
+            t.join()
+
+
 def test_main():
     suite = unittest.TestSuite()
     suite.addTest(DocTestSuite('_threading_local'))