<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def scan_file(self, src_file):
 """Scan candidate proxies from an existing file"""

        self.logger.info('start scanning file {} for proxy list...'
                         .format(src_file))
        with open(src_file, 'r') as fin:
            proxies = json.load(fin)
        for protocol in proxies.keys():
            for proxy in proxies[protocol]:
                self.proxy_queue.put({
                    'addr': proxy['addr'],
                    'protocol': protocol
                })