TextDocument: lazily convert substitution mapping to unicode

This way, the substitution mapping method can return a dynamic mapping,
the only requirement is to implement __getitem__

ne use case is to be able to use the same substitution script for
different CRM notification messages and only compute the "heavy"
substitution variables when they are actually needed.

See nexedi/erp5!1047

product/ERP5/Document/TextDocument.py

@@ -97,13 +97,15 @@ class TextDocument(CachedConvertableMixin, BaseConvertableFileMixin, TextContent
         if is_str:
           text = text.decode('utf-8')
 
-        unicode_mapping = {}
-        for k, v in mapping.iteritems():
+        class UnicodeMapping:
+          def __getitem__(self, item):
+            v = mapping[item]
             if isinstance(v, str):
               v = v.decode('utf-8')
             elif not isinstance(v, unicode):
               v = str(v).decode('utf-8')
-          unicode_mapping[k] = v
+            return v
+        unicode_mapping = UnicodeMapping()
 
         if safe_substitute:
           text = Template(text).safe_substitute(unicode_mapping)

product/ERP5/tests/testNotificationMessageModule.py

@@ -27,7 +27,7 @@
 #
 ##############################################################################
 
-
+import textwrap
 from Products.ERP5Type.tests.ERP5TypeTestCase import ERP5TypeTestCase
 from Products.ERP5Type.tests.utils import createZODBPythonScript
 
@@ -174,3 +174,28 @@ class TestNotificationMessageModule(ERP5TypeTestCase):
     self.assertRaises(KeyError, doc.convert, 'txt', safe_substitute=False)
     self.assertRaises(KeyError, doc.convert, 'html', safe_substitute=False)
     self.assertRaises(KeyError, doc.asSubjectText, safe_substitute=False)
+
+  def test_substitution_lazy_dict(self):
+    """Substitution script just needs to return an object implementing
+    __getitem__ protocol.
+    """
+    module = self.portal.notification_message_module
+    createZODBPythonScript(
+        self.portal, 'NotificationMessage_getDummySubstitionMapping', '**kw',
+        textwrap.dedent(
+            '''\
+            class DynamicDict:
+              def __getitem__(self, key):
+                return "(dynamic key: %s)" % key
+            return DynamicDict()
+            '''))
+    doc = module.newContent(
+        portal_type='Notification Message',
+        content_type='text/plain',
+        text_content='substitution text: ${a}',
+        text_content_substitution_mapping_method_id='NotificationMessage_getDummySubstitionMapping'
+    )
+
+    mime, text = doc.convert('txt')
+    self.assertEqual('text/plain', mime)
+    self.assertEqual('substitution text: (dynamic key: a)', text)