gh-130320: gettext: Test fallback translations

Lib/test/test_gettext.py

@@ -691,6 +691,66 @@ def test_cache(self):
         self.assertEqual(t.__class__, DummyGNUTranslations)
 
 
+class FallbackTranslations(gettext.NullTranslations):
+    def gettext(self, message):
+        return f'gettext: {message}'
+
+    def ngettext(self, msgid1, msgid2, n):
+        return f'ngettext: {msgid1}, {msgid2}, {n}'
+
+    def pgettext(self, context, message):
+        return f'pgettext: {context}, {message}'
+
+    def npgettext(self, context, msgid1, msgid2, n):
+        return f'npgettext: {context}, {msgid1}, {msgid2}, {n}'
+
+
+class FallbackTestCase(GettextBaseTest):
+    def test_null_translations_fallback(self):
+        t = gettext.NullTranslations()
+        t.add_fallback(FallbackTranslations())
+        self.assertEqual(t.gettext('foo'), 'gettext: foo')
+        self.assertEqual(t.ngettext('foo', 'foos', 1),
+                         'ngettext: foo, foos, 1')
+        self.assertEqual(t.pgettext('context', 'foo'),
+                         'pgettext: context, foo')
+        self.assertEqual(t.npgettext('context', 'foo', 'foos', 1),
+                         'npgettext: context, foo, foos, 1')
+
+    def test_gnu_translations_fallback(self):
+        with open(MOFILE, 'rb') as fp:
+            t = gettext.GNUTranslations(fp)
+        t.add_fallback(FallbackTranslations())
+        self.assertEqual(t.gettext('foo'), 'gettext: foo')
+        self.assertEqual(t.ngettext('foo', 'foos', 1),
+                         'ngettext: foo, foos, 1')
+        self.assertEqual(t.pgettext('context', 'foo'),
+                         'pgettext: context, foo')
+        self.assertEqual(t.npgettext('context', 'foo', 'foos', 1),
+                         'npgettext: context, foo, foos, 1')
+
+    def test_nested_fallbacks(self):
+        class NestedFallback(gettext.NullTranslations):
+            def gettext(self, message):
+                return 'fallback 1'
+
+        class NestedFallback2(gettext.NullTranslations):
+            def pgettext(self, context, message):
+                return 'fallback 2'
+
+        class NestedFallback3(gettext.NullTranslations):
+            def ngettext(self, msgid1, msgid2, n):
+                return 'fallback 3'
+
+        t = gettext.NullTranslations()
+        t.add_fallback(NestedFallback())
+        t.add_fallback(NestedFallback2())
+        t.add_fallback(NestedFallback3())
+        self.assertEqual(t.gettext('foo'), 'fallback 1')
+        self.assertEqual(t.pgettext('context', 'foo'), 'fallback 2')
+        self.assertEqual(t.ngettext('foo', 'foos', 1), 'fallback 3')
+
+
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
         support.check__all__(self, gettext,