Feed urls once

        Args:
            url_template: A string with parameters replaced with "{}".
            keyword: A string indicating the searching keyword.
            offset: An integer indicating the starting index.
            max_num: An integer indicating the max number of images to be crawled.
            page_step: An integer added to offset after each iteration.

def feed(self, url_template, keyword, offset, max_num, page_step):
        """Feed urls once

        Args:
            url_template: A string with parameters replaced with "{}".
            keyword: A string indicating the searching keyword.
            offset: An integer indicating the starting index.
            max_num: An integer indicating the max number of images to be crawled.
            page_step: An integer added to offset after each iteration.
        """
        for i in range(offset, offset + max_num, page_step):
            url = url_template.format(keyword, i)
            self.out_queue.put(url)
            self.logger.debug('put url to url_queue: {}'.format(url))