From 6e222dae5636f875c19ec66f730a4241abe33faa Mon Sep 17 00:00:00 2001
From: Carl Meyer <carl@oddbird.net>
Date: Sun, 6 Nov 2016 13:52:07 +0100
Subject: [PATCH] Fixed #27453 -- Avoided unnecessary recompilation of
 non-translated URL regexes.

---
 django/urls/resolvers.py                      | 49 ++++++++++++-------
 .../test_localeregexprovider.py               | 16 ++++++
 2 files changed, 48 insertions(+), 17 deletions(-)

diff --git a/django/urls/resolvers.py b/django/urls/resolvers.py
index 3c4f18c991..0bb5645dc1 100644
--- a/django/urls/resolvers.py
+++ b/django/urls/resolvers.py
@@ -79,6 +79,37 @@ def get_ns_resolver(ns_pattern, resolver):
     return RegexURLResolver(r'^/', [ns_resolver])
 
 
+class LocaleRegexDescriptor(object):
+    def __get__(self, instance, cls=None):
+        """
+        Return a compiled regular expression based on the active language.
+        """
+        if instance is None:
+            return self
+        # As a performance optimization, if the given regex string is a regular
+        # string (not a lazily-translated string proxy), compile it once and
+        # avoid per-language compilation.
+        if isinstance(instance._regex, six.string_types):
+            instance.__dict__['regex'] = self._compile(instance._regex)
+            return instance.__dict__['regex']
+        language_code = get_language()
+        if language_code not in instance._regex_dict:
+            instance._regex_dict[language_code] = self._compile(force_text(instance._regex))
+        return instance._regex_dict[language_code]
+
+    def _compile(self, regex):
+        """
+        Compile and return the given regular expression.
+        """
+        try:
+            return re.compile(regex, re.UNICODE)
+        except re.error as e:
+            raise ImproperlyConfigured(
+                '"%s" is not a valid regular expression: %s' %
+                (regex, six.text_type(e))
+            )
+
+
 class LocaleRegexProvider(object):
     """
     A mixin to provide a default regex property which can vary by active
@@ -91,23 +122,7 @@ class LocaleRegexProvider(object):
         self._regex = regex
         self._regex_dict = {}
 
-    @property
-    def regex(self):
-        """
-        Return a compiled regular expression based on the activate language.
-        """
-        language_code = get_language()
-        if language_code not in self._regex_dict:
-            regex = self._regex if isinstance(self._regex, six.string_types) else force_text(self._regex)
-            try:
-                compiled_regex = re.compile(regex, re.UNICODE)
-            except re.error as e:
-                raise ImproperlyConfigured(
-                    '"%s" is not a valid regular expression: %s' %
-                    (regex, six.text_type(e))
-                )
-            self._regex_dict[language_code] = compiled_regex
-        return self._regex_dict[language_code]
+    regex = LocaleRegexDescriptor()
 
     def describe(self):
         """
diff --git a/tests/urlpatterns_reverse/test_localeregexprovider.py b/tests/urlpatterns_reverse/test_localeregexprovider.py
index 9da8df656c..401e9a1ad0 100644
--- a/tests/urlpatterns_reverse/test_localeregexprovider.py
+++ b/tests/urlpatterns_reverse/test_localeregexprovider.py
@@ -5,6 +5,7 @@ import os
 from django.core.exceptions import ImproperlyConfigured
 from django.test import SimpleTestCase, mock, override_settings
 from django.urls import LocaleRegexProvider
+from django.urls.resolvers import LocaleRegexDescriptor
 from django.utils import translation
 from django.utils._os import upath
 
@@ -33,9 +34,24 @@ class LocaleRegexProviderTests(SimpleTestCase):
         self.assertEqual(de_compiled.pattern, '^foo-de/$')
         self.assertEqual(de_compiled, de_compiled_2)
 
+    def test_nontranslated_regex_compiled_once(self):
+        provider = LocaleRegexProvider('^foo/$')
+        with translation.override('de'):
+            de_compiled = provider.regex
+        with translation.override('fr'):
+            # compiled only once, regardless of language
+            error = AssertionError('tried to compile non-translated url regex twice')
+            with mock.patch('django.urls.resolvers.re.compile', side_effect=error):
+                fr_compiled = provider.regex
+        self.assertEqual(de_compiled.pattern, '^foo/$')
+        self.assertEqual(fr_compiled.pattern, '^foo/$')
+
     def test_regex_compile_error(self):
         """Regex errors are re-raised as ImproperlyConfigured."""
         provider = LocaleRegexProvider('*')
         msg = '"*" is not a valid regular expression: nothing to repeat'
         with self.assertRaisesMessage(ImproperlyConfigured, msg):
             provider.regex
+
+    def test_access_locale_regex_descriptor(self):
+        self.assertIsInstance(LocaleRegexProvider.regex, LocaleRegexDescriptor)