diff -r e7bd55db011b -r 52a4ad62b006 mercurial/bundle2.py
--- a/mercurial/bundle2.py	Fri Jan 15 16:16:25 2016 +0100
+++ b/mercurial/bundle2.py	Sat Jan 16 10:50:28 2016 -0500
@@ -851,13 +851,15 @@
                               self._advisoryparams, self._data, self.mandatory)
 
     # methods used to defines the part content
-    def __setdata(self, data):
+    @property
+    def data(self):
+        return self._data
+
+    @data.setter
+    def data(self, data):
         if self._generated is not None:
             raise error.ReadOnlyPartError('part is being generated')
         self._data = data
-    def __getdata(self):
-        return self._data
-    data = property(__getdata, __setdata)
 
     @property
     def mandatoryparams(self):