python - 用scrapy写一个spider，但是为什么 'yield item'不能在嵌套的for循环中工作？

Question

我有一个用 scrapy 编写的蜘蛛，但 yiled 项目未在 for 循环中执行，请参阅下面的代码。

def parse_paragraph(self, div_list, category_name, group_name):
    for div in div_list:
        duilian_text_list = div.xpath('./text()').extract()
        duilian_text_list = strip_list(duilian_text_list)
        if len(duilian_text_list) == 0:
            continue
        elif len(duilian_text_list) == 1:
            duilian_text = duilian_text_list[0]
            self.parse_duilian(duilian_text, category_name, group_name)
        elif len(duilian_text_list) == 2 and not is_single_line(duilian_text_list[0]):
            duilian_text = ''.join(duilian_text_list)
            self.parse_duilian(duilian_text, category_name, group_name)
        else:
            for duilian_text in duilian_text_list:
                duilian_item = DuilianItem()
                duilian_item['id'] = str(uuid.uuid4()).replace('-', '')
                duilian_item['category_id'] = getCategoryName(category_name)
                duilian_item['group_name'] = group_name
                duilian = parse_duilian(duilian_text)
                if duilian != '|':
                    duilian_item['name'] = duilian
                    duilian_item['desc'] = ''
                    duilian_item['author'] = ''
                    duilian_item['shuti'] = ''
                    duilian_item['word_count'] = len(duilian_item['name']) // 2
                    duilian_item['image_url'] = ''
                    print('-------I am here--------')
                    yield duilian_item

当我调用这个函数时，我在输出窗口中什么也没有得到，看起来yiled duilian_item行不起作用，它甚至阻止其他代码执行(上面的打印行)。

当我注释掉最后一行yiled duilian_item时，一切正常，我得到--------我在这里-------- 在输出窗口中，这里出了什么问题？

简单地说，下面的代码什么也不打印，但是如果我注释掉yiled 1，它会打印列表，所以Python中的yield在for循环中不起作用？

def strange_yield():
    list = [1, 2, 3]
    for i in list:
        print(i)
        yield 1

strange_yield()

Answer 1

当你在Python函数中使用yield时，该函数就变成了生成器函数。在您的 strange_yield 函数之后处理它的正确方法是:

my_yield = strange_yield()

my_yield 现在是生成器函数 strange_yield 的实例。生成器函数可以迭代，也可以使用 next() 函数获取下一个值:

print(next(my_yield))

或

for yield_value in my_yield:
  print(yield_value)