测试代码结构：

演示案例并没有进行网页爬取，主要目的是演示重复import的问题。

spider目录下是各个业务spider，把任务提交到crawler。

crawler中有个任务队列汇集各个业务spider提交的任务，然后在独立的线程中对任务进行实际的爬取动作。

main启动crawler及各个业务spider

在main.py和base_spider.py中导入crawler，由于导入的方式不一样，导致重复导入，而crawler也是不同的实列，导致爬取的任务队列一致为空

代码如下：

main.py

# main.py

# 这种引入方式与base_spider中引入的方式不一样，
# 会重复导入crawler实例，导致这里的crawler与base_spider中的crawler不是同一个实例
# from crawler import crawler
# 导入crawler实例的格式与base_spider中一样，保证只有一个crawler实例
from import_test.crawler import crawler
from spider.tb_pider import TbSpider

if __name__ == '__main__':
    crawler.run()
    tb_spider = TbSpider()
    tb_spider.run()


''' 导入crawler方式不一致的输出结果：
crawler run
crawler task length:  0
TbSpider add task:  1
TbSpider add task:  2
TbSpider add task:  3
TbSpider add task:  4
crawler task length:  0
crawler task length:  0
crawler task length:  0
crawler task length:  0
'''

''' 导入crawler方式一致的输出结果：
crawler run
crawler task length:  0
TbSpider add task:  1
TbSpider add task:  2
TbSpider add task:  3
TbSpider add task:  4
crawler task length:  4
4
crawler task length:  3
3
crawler task length:  2
2
crawler task length:  1
1
crawler task length:  0
crawler task length:  0
'''

crawler.py

# crawler.py
import threading
import time


class Crawler:
    def __init__(self):
        self._task_queue = []

    def length(self):
        return len(self._task_queue)

    def add_task(self, task):
        self._task_queue.append(task)

    def do_task(self):
        while True:
            print('crawler task length: ', self.length())
            if self.length() <= 0:
                time.sleep(2)
                continue
            print(self._task_queue.pop())

    def run(self):
        print('crawler run')
        crawler_thread = threading.Thread(target=self.do_task, name='crawler_thread')
        crawler_thread.start()


crawler = Crawler()

base_spider.py

# base_spider.py
from import_test.crawler import crawler


class BaseSpider:
    def __init__(self):
        self.crawler = crawler

    def run(self):
        raise NotImplementedError()

tb_spider.py

# tb_spider.py
from spider.base_spider import BaseSpider


class TbSpider(BaseSpider):

    def run(self):
        for i in range(1, 5):
            print('TbSpider add task: ', i)
            self.crawler.add_task(i)