stdout will be set to vim.message, but in early release of vim 7.3, message object didn't have method flush.

Wu, Fan · Wu, Fan · commit d2111cd3af07 · 2014-05-28T00:27:38.000-04:00
It will cause some problems in multiprocessing related code. Fixed of issue python-mode#426
diff --git a/pymode/rope.py b/pymode/rope.py
@@ -6,6 +6,7 @@
 import re
 import site
 import sys
+import StringIO
 
 from rope.base import project, libutils, exceptions, change, worder # noqa
 from rope.base.fscommands import FileSystemCommands # noqa
@@ -398,9 +399,12 @@ def _update_cache(importer, modules=None):
                 importer.generate_modules_cache(modules)
             importer.project.sync()
 
+        sys.stdout, stdout_ = StringIO.StringIO(), sys.stdout
+        sys.stderr, stderr_ = StringIO.StringIO(), sys.stderr
         process = multiprocessing.Process(target=_update_cache, args=(
             self.importer, modules))
         process.start()
+        sys.stdout, sys.stderr = stdout_, stderr_
 
 
 class ProgressHandler(object):
