Show proper error message for not allowed identifiers.

Due to interaction between Python(Scripts) and RestrictedPython,
following identifiers are not allowed:
- context
- container
- script
- traverse_subpath

Instead of throwing an "IndexError", now a proper error messsage
is shown.

This closes #33

modified: src/Products/PythonScripts/PythonScript.py
modified: src/Products/PythonScripts/tests/testPythonScript.py

src/Products/PythonScripts/PythonScript.py

@@ -122,7 +122,18 @@ class PythonScript(Script, Cacheable):
     ) + SimpleItem.manage_options + \
         Cacheable.manage_options
 
+    # following identifiers are not allowed due to interaction with
+    # RestrictedPython
+    bad_identifiers = [
+            'context', 'container', 'script', 'traverse_subpath'
+    ]
+
     def __init__(self, id):
+        if id in self.bad_identifiers:
+            raise ValueError(
+                "%s is not allowed as an identifier. "
+                "Please choose another name." % id
+                )
         self.id = id
         self.ZBindings_edit(defaultBindings)
         self._makeFunction()

src/Products/PythonScripts/tests/testPythonScript.py

@@ -272,6 +272,19 @@ def testAttributeAssignment(self):
                 func = self._newPS(defn + '\n' + asn % name)
                 self.assertRaises(TypeError, func)
 
+    def testBadIdentifiers(self):
+        """Some identifiers have to be avoided.
+
+        Background:
+        https://github.com/zopefoundation/Zope/issues/669
+        """
+        bad_identifiers = [
+            'context', 'container', 'script', 'traverse_subpath'
+            ]
+        for identifier in bad_identifiers:
+            with self.assertRaises(ValueError):
+                PythonScript(identifier)
+
 
 class TestPythonScriptGlobals(PythonScriptTestBase):