Google AppEngine URLFetch in Unit Tests

fetch_service_mock.py

from google.appengine.api import apiproxy_stub, apiproxy_stub_map, urlfetch
from google.appengine.ext import testbed
import unittest

class FetchServiceMock(apiproxy_stub.APIProxyStub):
    """
    Example::

        with FetchServiceMock(content='A MOCKERY', status_code=200) as fetch_mock:
            ret = main.method('amok')

            self.assertEquals('http://amok.com', fetch_mock.request.url())
            self.assertEquals('A MOCKERY', ret)

    Inspired by https://gist.github.com/kesor/1179782
    """
    def __init__(
            self, content='', status_code=500, headers=None,
            final_url=None, content_was_truncated=False):
        """
        :param final_url: URL after redirects.
        """
        # type: (basestring, int, Optional[Mapping], Optional[basestring], bool) -> None
        super(FetchServiceMock, self).__init__(testbed.URLFETCH_SERVICE_NAME)

        self.return_values = {
            'content': content,
            'statuscode': status_code,
            'headers': headers if headers else {},
            'finalurl': final_url,
            'contentwastruncated': content_was_truncated,
        }

    def __enter__(self):
        apiproxy_stub_map.apiproxy.ReplaceStub(
            testbed.URLFETCH_SERVICE_NAME, self)
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        complaining_mock = FetchServiceMock(
            content='Error: {} was reset. You must either mock urlfetch again '
            'or run Testbed.init_urlfetch_stub'.format(FetchServiceMock.__name__),
            status_code=418)
        apiproxy_stub_map.apiproxy.ReplaceStub(
            testbed.URLFETCH_SERVICE_NAME, complaining_mock)

    def _Dynamic_Fetch(self, request, response):
        rv = self.return_values
        response.set_content(rv['content'])
        response.set_statuscode(rv['statuscode'])
        for header_key, header_value in rv['headers'].iteritems():
            new_header = response.add_header()  # prototype for a header
            new_header.set_key(header_key)
            new_header.set_value(header_value)
        response.set_finalurl(rv['finalurl'] or request.url)
        response.set_contentwastruncated(rv['contentwastruncated'])

        # allow to query the object after it is used
        self.request = request
        self.response = response

class TestFetchServiceMock(unittest.TestCase):
    def setUp(self):
        self.testbed = testbed.Testbed()
        self.testbed.activate()
        self.testbed.init_urlfetch_stub()

    def test_mocking(self):
        with FetchServiceMock(self.testbed, 'A MOCKERY') as fetch_mock:
            fetched = urlfetch.fetch('http://httpstat.us/200')
            self.assertEqual(fetched.content, 'A MOCKERY')
            self.assertEqual(fetched.status_code, 500)

            self.assertEqual('http://httpstat.us/200', fetch_mock.request.url())

    def test_mock_resets(self):
        with FetchServiceMock(self.testbed, 'A MOCKERY'):
            active_stub = apiproxy_stub_map.apiproxy.GetStub(
                testbed.URLFETCH_SERVICE_NAME)
            self.assertIsInstance(active_stub, FetchServiceMock)

        active_stub = apiproxy_stub_map.apiproxy.GetStub(
            testbed.URLFETCH_SERVICE_NAME)
        self.assertNotIsInstance(active_stub, FetchServiceMock)

        # Enable to actually call the Internet
        if False:
            fetched2 = urlfetch.fetch('http://httpstat.us/200')
            self.assertEqual(fetched2.content, '200 OK')
            self.assertEqual(fetched2.status_code, 200)

Is it possible to set content type like 'application/json'?