refine error message for inconsistently failing asserts

--HG--
branch : trunk

py/magic/exprinfo.py

@@ -428,7 +428,9 @@ def interpret(source, frame, should_fail=False):
         import traceback
         traceback.print_exc()
     if should_fail:
-        return "(inconsistently failed then succeeded)"
+        return ("(assert failed but re-evaluating the assert expression "
+                "for printing intermediate values lead to a True value. "
+                "advise: avoid side-effects in assert expressions or use --nomagic)")
     else:
         return None
 

py/magic/testing/test_exprinfo.py

@@ -119,3 +119,15 @@ def test_keyboard_interrupt():
             pass
         else:
             raise AssertionError, "ex %s didn't pass through" %(exstr, )
+
+def test_inconsistent_assert_result(testdir):
+    p = testdir.makepyfile("""
+        def test_func():
+            def f(l=[1,0]): 
+                return l.pop()
+            assert f()
+    """)
+    result = testdir.runpytest(p)
+    s = result.stdout.str()
+    assert s.find("re-evaluating") 
+