POST data to Teamcity REST API url using urllib2

Hi,

I'm using Teamcity REST API to tag a build using following snippet of Python code. But I'm getting this message - TypeError: must be string or buffer, not dict

post_data = {"uri": "http://localhost:81/httpAuth/app/rest/builds/id:39/tags/", "name": "xxx"}
data = urllib.urlencode(post_data) 
request = urllib2.Request(url, data)

This is the curl command to tag a build that I'm trying to implement with urllib2.

curl -v -u user:password --request POST --header "Content-Type: application/xml" --data "<tags><tag name='xxx'/></tags>" "http://localhost:81/httpAuth/app/rest/builds/id:39/tags/