python 爬取 KeyError

Question

from html.parser import HTMLParser
import urllib.request
with urllib.request.urlopen('https://www.python.org/events/python-events/') as url:
    content = url.read()
html = content.decode('utf-8')

class MyHtmlParser(HTMLParser):
    def __init__(self):
        HTMLParser.__init__(self)
        self.location = []
        self.flag = False

    def handle_starttag(self, tag, attrs):
        if tag == 'span' and 'event-location' in dict(attrs)['class']:
            self.flag = True

    def handle_data(self, data):
        if self.flag:
            self.location.append(data)
            self.flag = False
parser = MyHtmlParser()
parser.feed(html)
for i in parser.location:
    print(i)
    

运行结果：

Traceback (most recent call last):
  File "C:/Users/user/PycharmProjects/untitled1/python6.py", line 22, in <module>
    parser.feed(html)
  File "C:\Users\user\AppData\Local\Programs\Python\Python35-32\lib\html\parser.py", line 111, in feed
    self.goahead(0)
  File "C:\Users\user\AppData\Local\Programs\Python\Python35-32\lib\html\parser.py", line 171, in goahead
    k = self.parse_starttag(i)
  File "C:\Users\user\AppData\Local\Programs\Python\Python35-32\lib\html\parser.py", line 345, in parse_starttag
    self.handle_starttag(tag, attrs)
  File "C:/Users/user/PycharmProjects/untitled1/python6.py", line 14, in handle_starttag
    if tag == 'span' and 'event-location' in dict(attrs)['class']:
KeyError: 'class'      

为什么会有KeyError呢？
这是爬取的目标网站的一段代码：

 <span class="event-location">Capital One McLean Conference Center in McLean, VA, USA</span> 

没有大小写错误，怎么会有KeyError呢？

另外我爬取这一段代码：

<a href="/events/python-events/461/">PyConES 2016 - Almeria</a> 

把代码改成if tag == 'a' and 'python-event' in dict(attrs)['href']:
就可以成功爬取。。。
所以是什么问题呢？
是class有禁忌？？？？

Answer 1

如果某个span没有 class属性，dict就报错~

        if tag == 'span' and 'event-location' in dict(attrs)['class']:
            self.flag = True

改成这样：

        if tag == 'span' and 'class' in dict(attrs) and 'event-location' in dict(attrs)['class']:
            self.flag = True

Answer 2

在python中，尽量不要用somedict['key']直接取值。而用somedict.get('key','default')这样，如果没有这个key,那么值为default