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.

2 files changed, 9 insertions, 3 deletions

diff --git a/django/utils/html.py b/django/utils/html.py
index 75eff0083d..825f139070 100644
--- a/django/utils/html.py
+++ b/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):
-        return text
+    if hasattr(text, '__html__'):
+        return text.__html__()
     else:
         return escape(text)
 
diff --git a/django/utils/safestring.py b/django/utils/safestring.py
index bec59385f7..aee6427b3c 100644
--- a/django/utils/safestring.py
+++ b/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):
     """