human
Solve the following python programming problem given the following code:
@ pytest.fixture 
def mock_thread() : 
    base_thrd = { 
        'id' : '5678', 
        'subject' : 'Test Thread', 
        'account_id' : MOCK_ACCOUNT_ID, 
        'object' : 'thread', 
        'folders' : [{ 
            'name' : 'inbox', 
            'display_name' : 'Inbox', 
            'id' : 'abcd', 
}], 
        'starred' : True, 
        'unread' : False, 
} 
    response_body = json.dumps (base_thrd) 
    def request_callback(request) : 
        payload = json.loads (request.body) 
        if ('folder' in payload) : 
            folder = { 
                'name' : 'test', 
                'display_name' : 'test', 
                'id' : payload ['folder'], 
} 
            base_thrd ['folders'] = [folder] 
return (200, { 
            
}, json.dumps (base_thrd)) 
endpoint = re.compile ((API_URL + '/threads/5678')) 
    responses.add (responses.GET, endpoint, content_type = 'application/json', status = 200, body = response_body) 
    responses.add_callback (responses.PUT, endpoint, content_type = 'application/json', callback = <MASK>) 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def request_callback(