Fixed #7261 -- support for __html__ for library interoperability

The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.

This is an updated patch based on jbalogh and ivank patches.

django/utils/html.py

@@ -65,8 +65,8 @@ def conditional_escape(text):
     """
     Similar to escape(), except that it doesn't operate on pre-escaped strings.
     """
-    if isinstance(text, SafeData):
+    if hasattr(text, '__html__'):
-        return text
+        return text.__html__()
     else:
         return escape(text)
 

django/utils/safestring.py

@@ -30,7 +30,13 @@ else:
     EscapeUnicode = EscapeText
 
 class SafeData(object):
-    pass
+    def __html__(self):
+        """
+        Returns the html representation of a string.
+
+        Allows interoperability with other template engines.
+        """
+        return self
 
 class SafeBytes(bytes, SafeData):
     """

tests/utils_tests/test_html.py

@@ -5,7 +5,7 @@ from datetime import datetime
 import os
 from unittest import TestCase
 
-from django.utils import html
+from django.utils import html, safestring
 from django.utils._os import upath
 from django.utils.encoding import force_text
 
@@ -192,3 +192,9 @@ class TestUtilsHtml(TestCase):
         self.assertEqual(quote('http://example.com/path/öäü/'), 'http://example.com/path/%C3%B6%C3%A4%C3%BC/')
         self.assertEqual(quote('http://example.com/%C3%B6/ä/'), 'http://example.com/%C3%B6/%C3%A4/')
         self.assertEqual(quote('http://example.com/?x=1&y=2'), 'http://example.com/?x=1&y=2')
+
+    def test_conditional_escape(self):
+        s = '<h1>interop</h1>'
+        self.assertEqual(html.conditional_escape(s),
+                         '&lt;h1&gt;interop&lt;/h1&gt;')
+        self.assertEqual(html.conditional_escape(safestring.mark_safe(s)), s)

tests/utils_tests/test_safestring.py

@@ -4,7 +4,7 @@ from django.template import Template, Context
 from django.test import TestCase
 from django.utils.encoding import force_text, force_bytes
 from django.utils.functional import lazy, Promise
-from django.utils.html import escape, conditional_escape
+from django.utils.html import escape
 from django.utils.safestring import mark_safe, mark_for_escaping
 from django.utils import six
 from django.utils import translation
@@ -50,3 +50,7 @@ class SafeStringTest(TestCase):
         s = mark_safe(translation.ugettext_lazy("username"))
         with translation.override('fr'):
             self.assertRenderEqual('{{ s }}', "nom d'utilisateur", s=s)
+
+    def test_html(self):
+        s = '<h1>interop</h1>'
+        self.assertEqual(s, mark_safe(s).__html__())