TextDocument: only call substitution method if it is used

One typical usage of substitution is Event_setTextContentFromNotificationMessage
which uses notification message to set the subject and the body of mail
message, which calls the substitution method twice, even if in most of
the cases the subject does not embed any substitution.

bt5/erp5_core_test/TestTemplateItem/portal_components/test.erp5.testNotificationMessageModule.py

@@ -172,6 +172,27 @@ class TestNotificationMessageModule(ERP5TypeTestCase):
     self.assertRaises(KeyError, doc.convert, 'html', safe_substitute=False)
     self.assertRaises(KeyError, doc.asSubjectText, safe_substitute=False)
 
+  def test_lazy_substitution(self):
+    """Tests that substitution method is not called when content does
+    not access any substitution variable.
+    """
+    module = self.portal.notification_message_module
+    createZODBPythonScript(self.portal,
+                           'NotificationMessage_getErrorSubstitutionMapping',
+                           '**kw',
+                           '''raise ValueError("should not be called")''')
+    doc = module.newContent(portal_type='Notification Message',
+                            title='title',
+                            content_type='text/plain',
+                            text_content='text_content',
+                            text_content_substitution_mapping_method_id=
+                            'NotificationMessage_getErrorSubstitutionMapping')
+
+    mime, text = doc.convert('txt')
+    self.assertEqual('text/plain', mime)
+    self.assertEqual('text_content', text.rstrip())
+    self.assertEqual('title', doc.asSubjectText())
+
   def test_substitution_lazy_dict(self):
     """Substitution script just needs to return an object implementing
     __getitem__ protocol.

product/ERP5/bootstrap/erp5_core/DocumentTemplateItem/portal_components/document.erp5.TextDocument.py

@@ -89,16 +89,25 @@ class TextDocument(CachedConvertableMixin, BaseConvertableFileMixin, TextContent
             ' mapping method %s from %r, so the content will not be'
             ' substituted.' % (method_id, self.getRelativeUrl()))
         return text
-      mapping = method(**kw)
 
       # unicode()
       is_str = isinstance(text, str)
       if six.PY2 and is_str:
         text = str2unicode(text)
 
-      class UnicodeMapping:
+      class LazyUnicodeMapping:
+        """Lazily calls the substitution method if some substitution is needed
+        and manage the str/unicode on py2.
+        """
+        _mapping = None
+        @property
+        def mapping(self):
+          if self._mapping is None:
+            self._mapping = method(**kw)
+          return self._mapping
+
         def __getitem__(self, item):
-          v = mapping[item]
+          v = self.mapping[item]
           if six.PY2:
             if isinstance(v, str):
               v = str2unicode(v)
@@ -108,7 +117,7 @@ class TextDocument(CachedConvertableMixin, BaseConvertableFileMixin, TextContent
             if not isinstance(v, str):
               v = str(v)
           return v
-      unicode_mapping = UnicodeMapping()
+      unicode_mapping = LazyUnicodeMapping()
 
       if safe_substitute:
         text = Template(text).safe_substitute(unicode_mapping)