urllib2 POST progress monitoring

I'm uploading a fairly large file with urllib2 to a server-side script via POST. I want to display a progress indicator that shows the current upload progress. Is there a hook or a callback provided by urllib2 that allows me to monitor upload progress? I know that you can do it with download using successive calls to the connection's read() method, but I don't see a write() method, you just add data to the request.