bs4 FeatureNotFound: Couldn't find a tree builder with the features you requested: lxml. Do you need to install a parser library?

安装beautifulsoup后，运行测试报错

from urllib import request
from bs4 import BeautifulSoup

url = "http://www.baidu.com"
rsp = request.urlopen(url)
content = rsp.read()

soup = BeautifulSoup(content, "lxml")

print(soup.title.string)

----------------------------------------------------------------------
FeatureNotFound                      Traceback (most recent call last)
~/data/spider/bs.py in <module>
     16 content = rsp.read()
     17 
---> 18 soup = BeautifulSoup(content, "lxml")
     19 
     20 print(soup.title.string)

~/data/spider/venv/lib/python3.7/site-packages/bs4/__init__.py in __init__(self, markup, features, builder, parse_only, from_encoding, exclude_encodings, **kwargs)
    196                     "Couldn't find a tree builder with the features you "
    197                     "requested: %s. Do you need to install a parser library?"
--> 198                     % ",".join(features))
    199             builder = builder_class()
    200             if not (original_features == builder.NAME or

FeatureNotFound: Couldn't find a tree builder with the features you requested: lxml. Do you need to install a parser library?

经过测试发现是lxml的问题，使用 pip install lxml 安装，安装完后，在运行还是一样出错, 经百度、知乎的说法是新的库不支持，新版本语法支持改变了

使用 pip install lxml时，自动安装的是最新 4.2.5版本

解决方法一:

　　卸载新的换老的就可以了

　　　　pip uninstall lxml

　　　　pip install lxml==3.7.0

方法二：

 在报错代码中把函数参数中所有的"lxml"改成"html.parser"
html.parser