human
Solve the following python programming problem given the following code:
def __init__(self, api_endpoint) : 
    '
        :param api_endpoint: name of the API end point
        ' 
    self.api_endpoint = api_endpoint 
    def request_callback(request, uri, headers) : 
        '
            :param request: HTTP request
            :param uri: URI/URL to send the request
            :param headers: header of the HTTP request
            :return: httpretty response
            ' 
        resp = json.loads (example_solr_response) 
        rows = int (request.querystring.get ('rows', [len (resp ['response'] ['docs'])]) [0]) 
        rows = min (rows, 300) 
        start = int (request.querystring.get ('start', [0]) [0]) 
        try : 
            resp ['response'] ['docs'] = resp ['response'] ['docs'] [start : (start + rows)] 
except IndexError : 
            resp ['response'] ['docs'] = resp ['response'] ['docs'] [start :] 
resp ['responseHeader'] ['params'] ['rows'] = rows 
        fl = request.querystring.get ('fl', ['id']) 
        resp ['response'] ['docs'] = [{ field : doc.get (field) for field in fl } for doc in resp ['response'] ['docs']] 
        resp ['responseHeader'] ['params'] ['fl'] = fl 
        return (200, headers, json.dumps (resp)) 
HTTPretty.register_uri (HTTPretty.GET, self.api_endpoint, body = <MASK>, content_type = 'application/json') 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def request_callback(