Backported issue #4589 to Python 2.5.3: Propagated an exception thrown by a

context manager's __exit__ method's result while it's being converted to bool.

Lib/test/test_with.py

@@ -505,6 +505,36 @@ class ExceptionalTestCase(unittest.TestCase, ContextmanagerAssertionMixin):
 
         self.assertRaises(GeneratorExit, shouldThrow)
 
+    def testErrorsInBool(self):
+        # issue4589: __exit__ return code may raise an exception
+        # when looking at its truth value.
+
+        class cm(object):
+            def __init__(self, bool_conversion):
+                class Bool:
+                    def __nonzero__(self):
+                        return bool_conversion()
+                self.exit_result = Bool()
+            def __enter__(self):
+                return 3
+            def __exit__(self, a, b, c):
+                return self.exit_result
+
+        def trueAsBool():
+            with cm(lambda: True):
+                self.fail("Should NOT see this")
+        trueAsBool()
+
+        def falseAsBool():
+            with cm(lambda: False):
+                self.fail("Should raise")
+        self.assertRaises(AssertionError, falseAsBool)
+
+        def failAsBool():
+            with cm(lambda: 1//0):
+                self.fail("Should NOT see this")
+        self.assertRaises(ZeroDivisionError, failAsBool)
+
 
 class NonLocalFlowControlTestCase(unittest.TestCase):
 

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.5.3?
 Core and builtins
 -----------------
 
+- Issue #4589: Propagated an exception thrown by a context manager's
+  __exit__ method's result while it's being converted to bool.
+
 - Issue #4317: Fixed a crash in the imageop.rgb2rgb8() function.
 
 - Issue #4230: If ``__getattr__`` is a descriptor, it now functions correctly.

Python/ceval.c

@@ -2263,9 +2263,16 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 			x = PyObject_CallFunctionObjArgs(x, u, v, w, NULL);
 			if (x == NULL)
 				break; /* Go to error exit */
-			if (u != Py_None && PyObject_IsTrue(x)) {
+			if (u != Py_None)
+				err = PyObject_IsTrue(x);
+			else
+				err = 0;
+			Py_DECREF(x);
+			if (err < 0)
+				break; /* Go to error exit */
+			else if (err > 0) {
+				err = 0;
 				/* There was an exception and a true return */
-				Py_DECREF(x);
 				x = TOP(); /* Again */
 				STACKADJ(-3);
 				Py_INCREF(Py_None);
@@ -2276,7 +2283,6 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 				Py_DECREF(w);
 			} else {
 				/* Let END_FINALLY do its thing */
-				Py_DECREF(x);
 				x = POP();
 				Py_DECREF(x);
 			}