为什么这里会说key有问题?但是print的时候却没有事?(bs4,attrs)

萌新,刚学python,老师说让搞个爬虫,结果这个问题卡了我好一会.在pycharm
写的.

代码如下:

def get_html(url):
    target = []
    headers = {
        'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36'}
    resp = requests.get(url, headers=headers)
    soup = BeautifulSoup(resp.text, 'lxml')
    target_undone =soup.find_all('a')
**     for a in target_undone:
         print(a['href'])
         target.append(a['href'])
     return target**

报错如下

/travels/nanjing9/3751226.html
/travels/nanjing9/3752303.html
/travels/nanjing9/1368968.html
/travels/nanjing9/2007739.html
/travels/nanjing9/1426946.html
/travels/nanjing9/1580737.html
/travels/nanjing9/2694051.html
/travels/nanjing9/1660560.html
/travels/nanjing9/2716240.html
/travels/nanjing9/1727375.html
Traceback (most recent call last):
  File "C:/Users/MoslemMaster/PycharmProjects/Scrapy/Testing.py", line 63, in <module>
    print(get_html('http://you.ctrip.com/travels/nanjing9/t3-d1.html'))
  File "C:/Users/MoslemMaster/PycharmProjects/Scrapy/Testing.py", line 16, in get_html
    print(a['href'])
  File "C:\Users\MoslemMaster\PycharmProjects\Scrapy\venv\lib\site-packages\bs4\element.py", line 1071, in __getitem__
    return self.attrs[key]
KeyError: 'href'

####难不成是在append的时候使用了其他函数么?似乎只要是赋值的时候都会出现这个问题,这是为啥啊?看着也是用了bs4库啊,为什么会提示key有问题?难不成attes字典被什么神秘力量给搞爆炸了?我尝试了 b = a['href'],结果同上,提示key
error.那么大致和赋值有关系了.那是为啥啊?

我就是个弱智,有些a里面没有href,我真是弱智