diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -12,11 +12,16 @@ import cmdutil, encoding
 import ui as uimod
 
 class request(object):
-    def __init__(self, args, ui=None, repo=None):
+    def __init__(self, args, ui=None, repo=None, fin=None, fout=None, ferr=None):
         self.args = args
         self.ui = ui
         self.repo = repo
 
+        # input/output/error streams
+        self.fin = fin
+        self.fout = fout
+        self.ferr = ferr
+
 def run():
     "run the command in sys.argv"
     sys.exit(dispatch(request(sys.argv[1:])))