Discuss another BBB problem (noticeable due to a test failure).

lib/python/Products/PageTemplates/tests/testExpressions.py

@@ -75,11 +75,40 @@ class ExpressionTests(unittest.TestCase):
         '''Test hybrid path expressions'''
         ec = self.ec
         assert ec.evaluate('x | python:1+1') == 2
+
+        # XXX The following test fails because int here is called
+        # which yields 0, not the int type. Why is it called? Because
+        # PathExpr calls everything that's not on the trees by the
+        # time it has counted to three.
         assert ec.evaluate('x | python:int') == int
+
+        # The whole expression is a PathExpr with two subexpressions:
+        # a SubPathExpr and a PythonExpr.  The first fails as
+        # intended, so the second is evaluated.  The result is called.
+        # The old PathExpr didn't do that.  Specifically, it wouldn't
+        # call builtin types (str, unicode, dict, list, tuple, bool).
+        # There are two things we can do about this now:
+
+        # a) Nothing. Accept the incompatibility from Zope 2.9 to
+        #    2.10.  Of course, this might break existing code, but
+        #    there are ways to make code compatible with 2.9 and 2.10:
+        #    Add a nocall: before the python: expression (if that's
+        #    possible?!?).  You just have to know about these things
+        #    beforehand.
+        
+        # b) Provide our own PathExpr implementation that does not
+        #    blindly call primitive types.  We would have to keep this
+        #    code in Zope 2 forever which means there'd be an
+        #    incompatibility between Zope 2 and Zope 3 ZPTs forever.
+
+        # I'm leaning towards option a). Given that this only turns
+        # out to be a problem with builtin types, the breakage is
+        # quite limited.
+
         assert ec.evaluate('x | string:x') == 'x'
         assert ec.evaluate('x | string:$one') == '1'
         assert ec.evaluate('x | not:exists:x')
-        
+
     def testWrappers(self):
         """Test if defer and lazy are returning their wrappers
         """