--- a/mercurial/bundle2.py	Sat May 24 16:08:05 2014 -0700
+++ b/mercurial/bundle2.py	Thu May 22 11:21:26 2014 -0700
@@ -554,13 +554,17 @@
 
     The part `type` is used to route the part to the application level
     handler.
+
+    The part payload is contained in ``part.data``. It could be raw bytes or a
+    generator of byte chunks. The data attribute cannot be modified after the
+    generation has begun.
     """
 
     def __init__(self, parttype, mandatoryparams=(), advisoryparams=(),
                  data=''):
         self.id = None
         self.type = parttype
-        self.data = data
+        self._data = data
         self.mandatoryparams = mandatoryparams
         self.advisoryparams = advisoryparams
         # status of the part's generation:
@@ -569,6 +573,15 @@
         # - True: generation done.
         self._generated = None
 
+    # methods used to defines the part content
+    def __setdata(self, data):
+        if self._generated is not None:
+            raise ReadOnlyPartError('part is being generated')
+        self._data = data
+    def __getdata(self):
+        return self._data
+    data = property(__getdata, __setdata)
+
     # methods used to generates the bundle2 stream
     def getchunks(self):
         if self._generated is not None:

--- a/tests/test-bundle2.t	Sat May 24 16:08:05 2014 -0700
+++ b/tests/test-bundle2.t	Thu May 22 11:21:26 2014 -0700
@@ -84,8 +84,9 @@
   >         bundler.newpart('b2x:replycaps', data=capsstring)
   > 
   >     if opts['pushrace']:
-  >         dummynode = '01234567890123456789'
-  >         bundler.newpart('b2x:check:heads', data=dummynode)
+  >         # also serve to test the assignement of data outside of init
+  >         part = bundler.newpart('b2x:check:heads')
+  >         part.data = '01234567890123456789'
   > 
   >     revs = opts['rev']
   >     if 'rev' in opts: