PEP 0492 -- Coroutines with async and await syntax. Issue #24017.

Lib/xml/dom/xmlbuilder.py

@@ -1,6 +1,7 @@
 """Implementation of the DOM Level 3 'LS-Load' feature."""
 
 import copy
+import warnings
 import xml.dom
 
 from xml.dom.NodeFilter import NodeFilter
@@ -331,13 +332,33 @@ class DOMBuilderFilter:
 del NodeFilter
 
 
+class _AsyncDeprecatedProperty:
+    def warn(self, cls):
+        clsname = cls.__name__
+        warnings.warn(
+            "{cls}.async is deprecated; use {cls}.async_".format(cls=clsname),
+            DeprecationWarning)
+
+    def __get__(self, instance, cls):
+        self.warn(cls)
+        if instance is not None:
+            return instance.async_
+        return False
+
+    def __set__(self, instance, value):
+        self.warn(type(instance))
+        setattr(instance, 'async_', value)
+
+
 class DocumentLS:
     """Mixin to create documents that conform to the load/save spec."""
 
-    async = False
+    async = _AsyncDeprecatedProperty()
+    async_ = False
 
     def _get_async(self):
         return False
+
     def _set_async(self, async):
         if async:
             raise xml.dom.NotSupportedErr(
@@ -363,6 +384,9 @@ class DocumentLS:
         return snode.toxml()
 
 
+del _AsyncDeprecatedProperty
+
+
 class DOMImplementationLS:
     MODE_SYNCHRONOUS = 1
     MODE_ASYNCHRONOUS = 2