Here are some code snippets from the test that I wrote for testing the REST API for one of our projects.
I am using appengine-rest-server as API provider on the server.

Do not copy these example verbatim, I’m only showing them as examples.
Each of this methods is defined inside of a class

Imports

from nose.tools import assert_equals, assert_true, assert_false
from restclient import GET, POST, PUT, DELETE
from lxml import etree, objectify
from StringIO import StringIO

Verifying XML against XML Schema

    def verify_against_schema(self, xml, schema):
        '''
        Check if data is valid for provided schema

        xml data to verify
        schema is url to the xml schema
        '''
        schema = etree.XMLSchema(etree.parse(StringIO(schema)))
        parser = etree.XMLParser(schema=schema)
        return etree.parse(StringIO(xml), parser=parser)

    def test_review_schema(self):
        'Vacation :: Verifying review schema'

        xml = '''
        
            4
            4
            Wedding
            4
            34r3423f3rf32f3f
            334323443
            300
            true
            2009-10-07
            Family
            4
        
        '''

        verified = self.verify_against_schema(xml, GET(self.url('/api/rest/metadata/Review')))
        assert_true(verified)

Creating an item

    def create_review(self, resort, full=False):
        'Create review for testing purposes'

        xml = '''
        
            4
            4
            Wedding
            4
            %s
            334323443
            300
            1
            2009-10-07
            Family
            4
        
        '''%resort

        if full:
            url = '/api/rest/Review?type=full'
        else:
            url = '/api/rest/Review'     

        return POST(self.url(url), body=xml, async=False, resp=True)

Updating an item

   def test_updating_review(self):
        'Vacation :: Updating review'

        response, resort = self.create_resort()
        response, review_id = self.create_review(resort)

        xml = '''
        
            5
            5
            Wedding
            5
            %s
            334323443
            520
            true
            2009-10-07
            Family
            4
        
        '''%resort
        response, content = PUT(self.url('/api/rest/Review/%s?type=full'%review_id), body=xml, resp=True, async=False )

        expected = '''
        
            %s
            5
            5
            Wedding
            5
            %s
            334323443
            520
            true
            2009-10-07
            Family
            4
        
        '''%(review_id,resort)

        expected = objectify.fromstring(expected)
        expected = etree.tostring(expected)

        result = objectify.fromstring(content)
        result = etree.tostring(result)

        assert_equals(expected, result)

I hope you find these helpful