How to mock object from third party library at specific time

I'm trying to mock the reponse from urllib2.urlopen from the Clarifay Python client.

The reason for this is that I don't want to expend an API call while testing, since I want to check that everything works ok but just fake the response to the server when I try to tag images using their client.

Problem is that urllib2.urlopen is used BEFORE I make use of their method to tag images (mostly to check API configuration, Oauth credential, etc) which kinda breaks my whole thing since I cannot go forward.

So this is my testing code:

    # Patch the actual code that clarifai client uses
    with patch('clarifai.client.mime_util.urllib2.urlopen') as mock_get:
        # Configure the mock to return a response with an OK status code.
        fd = open('fixtures/dummy_clarifai_result.json', 'r')
        dummy_response = json.load(fd)
        fd.close()
        mock_get.return_value = dummy_response
        response = self.tagger.process_images_clarifai(folder_name='sample_images')
        self.assertIsNotNone(response)
        self.assertTrue(response.empty)

The clarifai client calls the mime_util module to actually call this method, that I'm interested to mock the response

def post_multipart_request(url, multipart_message, headers={}):
    data, headers = message_as_post_data(multipart_message, headers)
    req = RequestWithMethod(url, 'POST', data, headers)
    f = urllib2.urlopen(req)
    response = f.read()
    f.close()
    return response

As I said, before the call to this method actually happens, the urlib2.urlopen is called in-between. Now I wonder if I can tell somehow to mock the object not the whole time but just when I want it...I really doubt it since just calling the tagging method executes this calling tree that in-between calls urlopen...

Also tried to mock directly that method (post_multipart_request) but I keep on getting errors with the path at the patch annotation...

Thanks!