beautifulsoup官方文档
html_doc = """
<html><head><title>The Dormouse's story</title></head>
<p class="title"><b>The Dormouse's story</b></p>
<p class="story">Once upon a time there were three little sisters; and their names were
<a href="/elsie" class="sister" >Elsie</a>,
<a href="/lacie" class="sister" >Lacie</a> and
<a href="/tillie" class="sister" >Tillie</a>;
and they lived at the bottom of a well.</p>
<p class="story">...</p>
"""
print(BeautifulSoup(html_doc, "", parse_only=only_a_tags).prettify())
# <a class="sister" href="/elsie" >
# Elsie
# </a>
# <a class="sister" href="/lacie" >
# Lacie
# </a>
# <a class="sister" href="/tillie" >
# Tillie
# </a>
print(BeautifulSoup(html_doc, "", parse_only=only_tags_with_id_link2).prettify())
# <a class="sister" href="/lacie" >
# Lacie
# </a>
print(BeautifulSoup(html_doc, "", parse_only=only_short_strings).prettify())
# Elsie
# ,
# Lacie
# and
# Tillie
# ...
#