Issue2378: pdb would delete free variables when stepping into a class statement.

The problem was introduced by r53954, the correction is to restore the symmetry between
PyFrame_FastToLocals and PyFrame_LocalsToFast

Lib/test/test_scope.py

@@ -519,6 +519,24 @@ self.assert_(X.passed)
         self.assert_("x" not in varnames)
         self.assert_("y" in varnames)
 
+    def testLocalsClass_WithTrace(self):
+        # Issue23728: after the trace function returns, the locals()
+        # dictionary is used to update all variables, this used to
+        # include free variables. But in class statements, free
+        # variables are not inserted...
+        import sys
+        sys.settrace(lambda a,b,c:None)
+        try:
+            x = 12
+
+            class C:
+                def f(self):
+                    return x
+
+            assert x == 12 # Used to raise UnboundLocalError
+        finally:
+            sys.settrace(None)
+
     def testBoundAndFree(self):
         # var is bound and free in class
 

Misc/NEWS

@@ -12,6 +12,10 @@ What's New in Python 2.6 beta 3?
 Core and Builtins
 -----------------
 
+- Issue #2378: An unexpected UnboundLocalError or NameError could appear when
+  the python debugger steps into a class statement: the free variables (local
+  variables defined in an outer scope) would be deleted from the outer scope.
+
 Library
 -------
 

Objects/frameobject.c

@@ -904,9 +904,12 @@ PyFrame_LocalsToFast(PyFrameObject *f, int clear)
 	if (ncells || nfreevars) {
 		dict_to_map(co->co_cellvars, ncells,
 			    locals, fast + co->co_nlocals, 1, clear);
-		dict_to_map(co->co_freevars, nfreevars,
-			    locals, fast + co->co_nlocals + ncells, 1, 
-			    clear);
+		/* Same test as in PyFrame_FastToLocals() above. */
+		if (co->co_flags & CO_OPTIMIZED) {
+			dict_to_map(co->co_freevars, nfreevars,
+			        locals, fast + co->co_nlocals + ncells, 1, 
+			        clear);
+		}
 	}
 	PyErr_Restore(error_type, error_value, error_traceback);
 }