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).

django/utils/text.py

@@ -7,7 +7,9 @@ from io import BytesIO
 
 from django.utils import six
 from django.utils.encoding import force_text
-from django.utils.functional import SimpleLazyObject, keep_lazy, keep_lazy_text
+from django.utils.functional import (
+    SimpleLazyObject, keep_lazy, keep_lazy_text, lazy,
+)
 from django.utils.safestring import SafeText, mark_safe
 from django.utils.six.moves import html_entities
 from django.utils.translation import pgettext, ugettext as _, ugettext_lazy
@@ -434,3 +436,12 @@ def camel_case_to_spaces(value):
     trailing whitespace.
     """
     return re_camel_case.sub(r' \1', value).strip().lower()
+
+
+def _format_lazy(format_string, *args, **kwargs):
+    """
+    Apply str.format() on 'format_string' where format_string, args,
+    and/or kwargs might be lazy.
+    """
+    return format_string.format(*args, **kwargs)
+format_lazy = lazy(_format_lazy, six.text_type)

docs/ref/utils.txt

@@ -868,6 +868,26 @@ appropriate entities.
 .. module:: django.utils.text
     :synopsis: Text manipulation.
 
+.. function:: format_lazy(format_string, *args, **kwargs)
+
+    .. versionadded:: 1.11
+
+    A version of :meth:`str.format` for when ``format_string``, ``args``,
+    and/or ``kwargs`` contain lazy objects. The first argument is the string to
+    be formatted. For example::
+
+        from django.utils.text import format_lazy
+        from django.utils.translation import pgettext_lazy
+
+        urlpatterns = [
+            url(format_lazy(r'{person}/(?P<pk>\d+)/$', person=pgettext_lazy('URL', 'person')),
+                PersonDetailView.as_view()),
+        ]
+
+    This example allows translators to translate part of the URL. If "person"
+    is translated to "persona", the regular expression will match
+    ``persona/(?P<pk>\d+)/$``, e.g. ``persona/5/``.
+
 .. function:: slugify(allow_unicode=False)
 
     Converts to ASCII if ``allow_unicode`` is ``False`` (default). Converts spaces to

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)