Added a flush method to the SubSocketChannel. The Qt console frontend now uses this method to ensure that output has been processed before it writes a new prompt.

IPython/frontend/qt/console/frontend_widget.py

@@ -267,6 +267,10 b' class FrontendWidget(HistoryConsoleWidget):'
         self.appendPlainText(omsg['content']['data'])
         
     def _handle_execute_reply(self, rep):
+        # Make sure that all output from the SUB channel has been processed
+        # before writing a new prompt.
+        self.kernel_manager.sub_channel.flush()
+
         content = rep['content']
         status = content['status']
         if status == 'error':

IPython/frontend/qt/kernelmanager.py

@@ -47,6 +47,12 b' class QtSubSocketChannel(SubSocketChannel, QtCore.QObject):'
         elif msg_type in ('pyerr', 'stderr'):
             self.error_received.emit(msg)
 
+    def flush(self):
+        """ Reimplemented to ensure that signals are dispatched immediately.
+        """
+        super(QtSubSocketChannel, self).flush()
+        QtCore.QCoreApplication.instance().processEvents()
+
 
 class QtXReqSocketChannel(XReqSocketChannel, QtCore.QObject):
 

IPython/zmq/kernelmanager.py

@@ -7,6 +7,7 b' TODO: Create logger to handle debugging and console messages.'
 # Standard library imports.
 from Queue import Queue, Empty
 from threading import Thread
+import time
 import traceback
 
 # System library imports.
@@ -107,8 +108,24 b' class SubSocketChannel(ZmqSocketChannel):'
         self.handlers[msg_type] = callback
 
     def remove_handler(self, msg_type):
+        """Remove the callback for msg type."""
         self.handlers.pop(msg_type, None)
 
+    def flush(self):
+        """Immediately processes all pending messages on the SUB channel. This
+           method is thread safe.
+        """
+        self._flushed = False
+        self.ioloop.add_callback(self._flush)
+        while not self._flushed:
+            time.sleep(0)
+        
+    def _flush(self):
+        """Called in this thread by the IOLoop to indicate that all events have
+           been processed.
+        """
+        self._flushed = True
+
 
 class XReqSocketChannel(ZmqSocketChannel):