Fixed #26866 -- Added format_lazy function

Added format_lazy function to django.utils.text module.
Useful when dealing with relative complex lazy string concatenations
(e.g. in urls.py when translating urls in regular expressions).

tests/utils_tests/test_text.py

@@ -6,7 +6,8 @@ import json
 from django.test import SimpleTestCase
 from django.utils import six, text
 from django.utils.functional import lazystr
-from django.utils.translation import override
+from django.utils.text import format_lazy
+from django.utils.translation import override, ugettext_lazy
 
 IS_WIDE_BUILD = (len('\U0001F4A9') == 1)
 
@@ -225,3 +226,24 @@ class TestUtilsText(SimpleTestCase):
         out = text.compress_sequence(seq)
         compressed_length = len(b''.join(out))
         self.assertTrue(compressed_length < actual_length)
+
+    def test_format_lazy(self):
+        self.assertEqual('django/test', format_lazy('{}/{}', 'django', lazystr('test')))
+        self.assertEqual('django/test', format_lazy('{0}/{1}', *('django', 'test')))
+        self.assertEqual('django/test', format_lazy('{a}/{b}', **{'a': 'django', 'b': 'test'}))
+        self.assertEqual('django/test', format_lazy('{a[0]}/{a[1]}', a=('django', 'test')))
+
+        t = {}
+        s = format_lazy('{0[a]}-{p[a]}', t, p=t)
+        t['a'] = lazystr('django')
+        self.assertEqual('django-django', s)
+        t['a'] = 'update'
+        self.assertEqual('update-update', s)
+
+        # The format string can be lazy. (string comes from contrib.admin)
+        s = format_lazy(
+            ugettext_lazy("Added {name} \"{object}\"."),
+            name='article', object='My first try',
+        )
+        with override('fr'):
+            self.assertEqual('article «\xa0My first try\xa0» ajouté.', s)