python首次使用generator时遇到的问题

1. 因为刚开始开发python程序不久就遇到了一个generator的问题，python有的特性还是和java有所区别，记录一下这个问题

   def parse(self, response):
        self.logger.info("smzdm_jingxuan spider starting")
        self.broswer.get('https://www.smzdm.com/jingxuan/')
        last_height = self.broswer.execute_script("return document.body.scrollHeight")
        print(last_height)
        count = 0

        while True:
            if count == 30:
                break
            self.broswer.execute_script("window.scrollTo(0, document.body.scrollHeight);")
            time.sleep(2)
            new_height = self.broswer.execute_script("return document.body.scrollHeight")
            if new_height == last_height:
                break
            last_height = new_height
            time.sleep(1.2)
            count = count + 1

        source = self.broswer.page_source
        self.logger.info('before parse goods in page')
        try:
            # return self.parse_goods_in_page(source)
            self.parse_goods_in_page(source)
        except Exception as e:   def parse_goods_in_page(self, source):

   def parse_goods_in_page(self, source):
        print('parse_goods_in_page in')
        self.logger.info("parse_goods_in_page in in")
        scrapy_selector = Selector(text=source)
        items_selector = scrapy_selector.xpath('//div[@class="z-feed-content"]')
        self.logger.info('Theres a total of ' + str(len(items_selector)) + ' links.')
        s = 0

        for item_selector in items_selector:
            try:
                self.logger.info("s=" + str(s))
                .......
                .......
                s = s + 1
                yield smzdm_item


            except Exception as e:
                self.logger.info('Reached last iteration #' + e.__traceback__ + str(s))

        self.broswer.close()
        return

现象是在parse()中调parse_goods_in_page(),发现调不到，打断点跟踪也进不去方法。因为java没有这样的特性所以一开始有点懵。后来查了资料，在《Python Cookbook》中有说到

“The mere presence of the yield statement in a function turns it into a generator. Unlike a normal function, a generator only runs in response to iteration”

Excerpt From: David Beazley and Brian K. Jones. “Python Cookbook.” Apple Books. 

也就是说，关键字yield说明parse_goods_in_page这个方法是个generator。而直接调用generator方法，如代码中 self.parse_goods_in_page(source)这种不会运行这个方法，而只是返回了一个generator。如 gen = self.parse_goods_in_page(source), 可以看到里面的日志没有输出。要运行的话只能next(gen),此时代码会跑到 yield smzdm_item 然后返回，如果再运行一次 next(gen),代码从yield smzdm_item后继续第二次循环到yield smzdm_item返回。

那我想调用parse_goods_in_page这个方法希望它一下子整个执行完怎么办呢，可以把self.parse_goods_in_page(source)这句改成yield from self.parse_goods_in_page(source)[https://www.python.org/dev/peps/pep-0380/]或 return self.parse_goods_in_page(source)

2. 需要注意以下代码

   def parse_goods_in_page(self, page_items_selector_list):
  ...
                    for t in create_time_text:
                        if t.strip():
                            item_create_time = t.strip()
                            self.logger.info("create_time = " + item_create_time)
                            smzdm_item['item_create_time'] = item_create_time
                    # 拼多多的create_time和其他的不太一样
                    self.logger.info("item_create_time = " + str(item_create_time))
                    if not item_create_time:
                        create_time_text = item_selector.css(".wechat-time::text").get()
                        self.logger.info("pdd create_time_text = " + str(create_time_text))
                        item_create_time = create_time_text.strip()
                        self.logger.info("create_time = " + item_create_time)

             ........
                    s = s + 1
                    yield smzdm_item

在开发过程中发现item_create_time没有赋值的情况下（没进入到for t in create_time_text），执行到if not item_create_time时没有报错（变量未被赋值时这个变量应该不存在）,打印出来后确实有值。注意到这个方法是个generator,那前面next跑了一次，接下来next时这个item_create_time其实是上一次的值。所以要避免这个问题在方法前面可以先给item_create_time初始化一下