Don't quietly use English strings when we fail to load translation file.

Fixes #177.

chstrings/__init__.py

@@ -159,11 +159,16 @@ def _load_strings_for_lang_tag(lang_tag):
         return json.load(json_fp)
 
 def get_localized_strings(config, lang_tag):
-    strings = _load_strings_for_lang_tag(config.fallback_lang_tag)
+    localized_strings = {}
     try:
-        strings.update(_load_strings_for_lang_tag(lang_tag))
+        localized_strings = _load_strings_for_lang_tag(lang_tag)
     except:
-        pass
+        return localized_strings
+
+    # Complete the strings with the fallback in case of incomplete
+    # translations, but only if there is a translation at all.
+    strings = _load_strings_for_lang_tag(config.fallback_lang_tag)
+    strings.update(localized_strings)
     strings = _preprocess_variables(config, strings)
     _partition_js_strings(strings)
     return strings

chstrings/chstrings_test.py

@@ -43,6 +43,15 @@ def _load_strings_side_effect(lang_tag):
             self.assertEqual(fallback_strings['instructions_goal'],
                 strings['instructions_goal'])
 
+    def test_missing_lang_tag_has_no_fallback(self):
+        # We must only apply the fallback strings if there is an incomplete
+        # strings file (test_fallback_lang_tag exercises that behavior).
+        # If the file does not exist at all, we must return {} rather than just
+        # the fallback strings.
+        self.assertEqual(
+            chstrings.get_localized_strings(cfg, 'fake'), {})
+
+
 if __name__ == '__main__':
     for lc in config.LANG_CODES_TO_LANG_NAMES:
         cfg = config.get_localized_config(lc, api = False)

handlers/common.py

@@ -56,7 +56,7 @@ def load_strings_for_request(lang_code, cfg, accept_language_hdr):
     header_locales = accept_language_hdr.split(',')
     for l in header_locales:
         lang_tag = l.split(';', 1)[0]  # drop weight, if any
-        lang = l.split('-', 1)[0] # en-GB -> en
+        lang = l.split('-', 1)[0]  # en-GB -> en
 
         lang_tag_matches_config = (
             lang == lang_code or