from urllib.request import urlopen
from bs4 import BeautifulSoup
html = urlopen("http://www.pythonscraping.com/pages/page1.html")
bsObj = BeautifulSoup(html.read())
print(bsObj.h2)

代码运行之后警告如下：
UserWarning: No parser was explicitly specified, so I'm using the best available HTML parser for this system ("lxml"). This usually isn't a problem, but if you run this code on another system, or in a different virtual environment, it may use a different parser and behave differently.

The code that caused this warning is on line 4 of the file D:/Python/venv/test8.py. To get rid of this warning, pass the additional argument 'features="lxml"' to the BeautifulSoup constructor.

翻译如下：
用户警告：没有显式指定语法分析器，因此我使用了此系统的最佳可用HTML语法分析器（“lxml”）。这通常不是问题，但是如果您在另一个系统上运行此代码，或者在不同的虚拟环境中运行此代码，它可能会使用不同的解析器并表现出不同的行为。

导致此警告的代码位于文件d:/python/venv/test8.py的第4行。要消除此警告，请将附加参数'features=“lxml”'传递给beautifulsoup构造函数。

解决：指定解析器，一般使用'lxml'

from urllib.request import urlopen
from bs4 import BeautifulSoup
html = urlopen("http://www.pythonscraping.com/pages/page1.html")
bsObj = BeautifulSoup(html.read(),'lxml')
print(bsObj.h2)