BeautifulSoup-使用指北-0x02操作解析树

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在解析树中导航

参考: Navigating the tree

在学习与解析树相关的 ” 导航字段 ” 之前，我们需要先了解 BeautifulSoup 解析树的结构，下面这段 HTML 和其解析树如下:

markup = '''
<p>To find out
    <em>more</em> see the
    <a href="http://www.w3.org/XML">standard</a>.
</p>'''soup = BeautifulSoup(markup,'lxml')

⚠” 导航字段 ” 的返回值总是节点对象(如，Tag 对象、NavigableString 对象)，或由节点对象组成的列表(或迭代器)。

Going down

Tag 中包含的字符串或 Tag 等节点被视作该 Tag 的 children (或 descendants)节点。为了便于在 children (或 descendants)节点中进行导航，BeautifulSoup 提供了许多与此相关的方法。

⚠BeautifulSoup 中的字符串节点 (如，NavigableString 和注释) 不支持与导航相关的属性，因为字符串节点永远不会包含任何 children 节点。

节点名

可使用节点名来选取目标节点，此时会返回子孙节点中的第一个同名节点。

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <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="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from bs4 import BeautifulSoup
soup = BeautifulSoup(html_doc, 'html.parser')
print(repr(f"{type(soup.head)}:{soup.head}"))
print(repr(f"{type(soup.title)}:{soup.title}"))
print(repr(f"{type(soup.a)}:{soup.a}"))

输出:

"<class'bs4.element.Tag'>:<head>\n<title>The Dormouse's story</title>\n</head>""<class'bs4.element.Tag'>:<title>The Dormouse's story</title>"'<class \'bs4.element.Tag\'>:<a class="sister"href="http://example.com/elsie"id="link1">Elsie</a>'

.contents????

.contents 字段会返回一个由 ” 直接子节点 ” 组成的列表:

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <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="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
p = soup.find('p', 'story')
pprint(p.contents)
pprint([type(i) for i in p.contents])

输出:

['Once upon a time there were three little sisters; and their names were\n'
 ' ',
 <a class="sister" href="http://example.com/elsie" id="link1">Elsie</a>,
 ',\n',
 <a class="sister" href="http://example.com/lacie" id="link2">Lacie</a>,
 'and\n',
 <a class="sister" href="http://example.com/tillie" id="link3">Tillie</a>,
 ';\n        and they lived at the bottom of a well.\n']
[<class 'bs4.element.NavigableString'>,
 <class 'bs4.element.Tag'>,
 <class 'bs4.element.NavigableString'>,
 <class 'bs4.element.Tag'>,
 <class 'bs4.element.NavigableString'>,
 <class 'bs4.element.Tag'>,
 <class 'bs4.element.NavigableString'>]

⚠.contents 返回的列表中的元素是节点对象，不是字符串对象。

⚠BeautifulSoup 中的字符串节点 (如，NavigableString 和注释) 不支持 .contents 字段，因为字符串节点永远不会包含任何 children 节点，强行获取会抛出异常:

soup = BeautifulSoup(html_doc, 'html.parser')
pprint(soup.title.contents[0].contents)
#> AttributeError: 'NavigableString' object has no attribute 'contents'

.children????

.children 是 .contents 的迭代器版本，源代码如下:

#Generator methods
@property
def children(self):
    # return iter() to make the purpose of the method clear
    return iter(self.contents)  # XXX This seems to be untested.

.descendants????

.descendants 字段会返回一个包含 ” 所有子孙节点 ” 的生成器，从而允许你以递归方式遍历当前节点的所有子孙节点。

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <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="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.head.descendants)
print(list(soup.head.descendants))

输出:

<generator object Tag.descendants at 0x000001D502BA2750>
['\n', <title>The Dormouse's story</title>,"The Dormouse's story", '\n']

.string????

.string 属性被用于获取 tag 内部的字符串，其返回值可以是 NavigableString , None , Comment，具体如下:

  • 如果 tag 仅含一个字符串子项，则返回一个包含该字符串的 NavigableString 对象:

    from bs4 import BeautifulSoup
soup = BeautifulSoup('<b class="boldest">Extremely bold</b>', 'lxml')
tag = soup.b
print(type(tag.string))
#> <class 'bs4.element.NavigableString'>
print(tag.string)
#> Extremely bold

  • 如果 tag 中仅包含一个子 tag，且该 tag 仅含一个字符串子项，则返回一个包含该字符串的 NavigableString 对象，该逻辑可递归:

    soup = BeautifulSoup('<b class="boldest">
                         <i>
                           <i>Extremely bold</i>
                         </i></b>','lxml')
tag = soup.b
print(type(tag.string))
#> <class 'bs4.element.NavigableString'>
print(tag.string)
#> Extremely bold

  • 如果 tag 中没有子项，或单个子项中不包含字符串，或有多个子项，或有多个字符串子项，都将会返回 None:

    # 没有子项
soup = BeautifulSoup('<b class="boldest"></b>', 'lxml')
tag = soup.b
print(type(tag.string))
#> <class 'NoneType'>
print(tag.string)
#> None

# 子项中不包含字符串
soup = BeautifulSoup('<b class="boldest"><i></i></b>', 'lxml')
print(soup.b.string)
#> None

# 多个子项, 即便包含字符串也返回 None
soup = BeautifulSoup('<b class="boldest">link to <i>example.com</i></b>',
                     'lxml')
print(soup.b.string)
#> None

  • 如果 tag 仅含一个注释子项，则返回一个包含该注释的 Comment 对象:

    from bs4 import BeautifulSoup
markup = "<b><!--Hey, buddy. Want to buy a used parser?--></b>"
soup = BeautifulSoup(markup, 'lxml')
comment = soup.b.string
print(type(comment))
#> <class 'bs4.element.Comment'>
print(comment)
#> Hey, buddy. Want to buy a used parser?

.strings????

如果 tag 有数个内含字符串的子孙节点，.stirng 字段允许你以递归方式遍历这些字符串:

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <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="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.strings)
pprint(list(soup.strings))

输出:

<generator object Tag._all_strings at 0x0000013C23342750>
['\n',
 '\n',
 '\n',
 "The Dormouse's story",'\n','\n','\n',"The Dormouse's story",
 '\n',
 'Once upon a time there were three little sisters; and their names were\n'
 '','Elsie',',\n        ','Lacie',' and\n        ','Tillie',';\n        and they lived at the bottom of a well.\n    ','\n','...','\n']

stripped_strings????

.stripped_strings 的功能与 .strings 类似，但会剥离掉多余的空白符。.stripped_strings 会忽略掉完全由空白符组成的字符串，并删除字符串开头和结尾处的空白符。

from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.stripped_strings)
pprint(list(soup.stripped_strings))

输出:

<generator object Tag.stripped_strings at 0x000002644BE22750>
["The Dormouse's story","The Dormouse's story",
 'Once upon a time there were three little sisters; and their names were',
 'Elsie',
 ',',
 'Lacie',
 'and',
 'Tillie',
 ';\n        and they lived at the bottom of a well.',
 '...']

Going up

每个 tag 或字符串都有父节点: 包含当前 tag 的节点。

.parent????

.parent 字段用于访问当前节点的父节点。

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <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="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.parent)
print(soup.html.parent.name)
print(soup.title.parent.name)

输出:

None
[document]
head

.parents????

.parent 字段会返回一个内含所有祖先节点的生成器，可用于迭代访问当前节点的所有祖先节点:

from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
link = soup.a
print(link.parents)
print([i.name for i in link.parents])

输出:

<generator object PageElement.parents at 0x0000013D87571750>
['p', 'body', 'html', '[document]']

Going sideways

先考虑下面这个示例:

sibling_soup = BeautifulSoup("<a><b>text1</b><c>text2</c></b></a>",
                             'html.parser')
print(sibling_soup.prettify())

输出:

<a>
 <b>
  text1
 </b>
 <c>
  text2
 </c>
</a>

<b> 和 <c> 是兄弟节点，因为它们拥有相同的父节点；字符串 'text1' 和 'text2' 不是兄弟节点，因为它们的父节点不同。

.next_sibling????.previous_sibling????

.next_sibling 字段用于选取下一个兄弟节点，.previous_sibling 字段用于选取上一个兄弟节点:

sibling_soup = BeautifulSoup("<a><b>text1</b><c>text2</c></b></a>",
                             'html.parser')
print(sibling_soup.b.previous_sibling)
print(sibling_soup.b.next_sibling)

print(sibling_soup.c.previous_sibling)
print(sibling_soup.c.next_sibling)

输出:

None
<c>text2</c>
<b>text1</b>
None

<c> 没有 .next_sibling，因为在 <c> 之后并没有兄弟节点；<b> 没有 .previous_sibling，因为在 <b> 之前并没有兄弟节点。

⚠在实际的文档中，节点的 .next_sibling (或 .previous_sibling) 字段可能是包含空白符的字符串:

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <p class="title"><b>The Dormouse's story</b></p>
    <b>The</b>
    <p class="story">Once upon a time there were three little sisters; and their names were
        <a href="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(repr(soup.a.next_sibling))

输出:

',\n'

.next_siblings????.previous_siblings????

.next_siblings 和 .previous_siblings 会返回由兄弟节点组成的生成器:

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <p class="title"><b>The Dormouse's story</b></p>
    <b>The</b>
    <p class="story">Once upon a time there were three little sisters; and their names were
        <a href="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(soup.a.next_siblings)
pprint([repr(i) for i in soup.a.next_siblings])

pprint([repr(i) for i in soup.find(id='link3').previous_siblings])

输出:

<generator object PageElement.next_siblings at 0x000001DDDD0C2750>
["',\\n'",
 '<a class="sister"href="http://example.com/lacie"id="link2">Lacie</a>',
 "'and\\n'",
 '<a class="sister"href="http://example.com/tillie"id="link3">Tillie</a>',
 "';\\n        and they lived at the bottom of a well.\\n'"]
["'and\\n'",
 '<a class="sister"href="http://example.com/lacie"id="link2">Lacie</a>',
 "',\\n'",
 '<a class="sister"href="http://example.com/elsie"id="link1">Elsie</a>',
 "'Once upon a time there were three little sisters; and their names"
 "were\\n'"]

Going back and forth

先看一段 “three sisters” 中的 HTML 文档:

<html><head><title>The Dormouse's story</title></head>
<p class="title"><b>The Dormouse's story</b></p>

HTML 解析器在获得上面的 HTML 文档后，会将其转换成一连串事件: “ 打开 <html> 标签 ”，” 打开一个 <head> 标签 ”，” 打开一个 <title> 标签 ”，” 添加一段字符串 ”，” 关闭 <title> 标签 ”，” 打开 <p> 标签 ”，等等。BeautifulSoup 提供了重现文档初始解析过程的工具。

.next_element????.previous_element????

.next_element 字段指向下一个被解析的节点，其结果通常与 .next_sibling 不同:

html_doc = """
<html>
<head>
    <title>The Dormouse's story</title>
</head>
<body>
    <p class="title"><b>The Dormouse's story</b></p>
    <b>The</b>
    <p class="story">Once upon a time there were three little sisters; and their names were
        <a href="http://example.com/elsie" class="sister" id="link1">Elsie</a>,
        <a href="http://example.com/lacie" class="sister" id="link2">Lacie</a> and
        <a href="http://example.com/tillie" class="sister" id="link3">Tillie</a>;
        and they lived at the bottom of a well.
    </p>

    <p class="story">...</p>
"""
from pprint import pprint
from bs4 import BeautifulSoup

soup = BeautifulSoup(html_doc, 'html.parser')
print(repr(soup.find('a', id='link3').next_sibling)) # 下一个兄弟节点
print(repr(soup.find('a', id='link3').next_element)) # 下一个被解析的节点

输出:

';\n        and they lived at the bottom of a well.\n'
'Tillie'

.previous_element 字段指向前一个被解析的节点，其结果通常与 .previous_sibling 不同:

sibling_soup = BeautifulSoup("<a><b>text1</b><c>text2</c></b></a>",
                             'html.parser')

print(repr(sibling_soup.c.next_element))
print(repr(sibling_soup.c.next_sibling))

输出:

'text2'
None

.next_elements????.previous_elements????

.next_elements 会返回一个生成器，该生成器会按照解析顺序逆向获取先前解析的节点；.previous_elements 会返回一个生成器，该生成器会按照解析顺序依次获取之后解析的节点。

sibling_soup = BeautifulSoup("<a><b>text1</b><c>text2</c></b></a>",
                             'html.parser')

pprint([repr(i) for i in sibling_soup.a.next_elements])
print(repr(sibling_soup.c.next_sibling))

修改解析树

GitHub@orca-j35，所有笔记均托管于 python_notes 仓库

BeautifulSoup 的强项是搜索文档树，但是你也可以利用 BeautifulSoup 来修改文档树，并将修改后的文档树保存到一个新的 HTML 或 XML 文档中，具体功能如下:

  • 修改 tag 名和属性
  • 修改 .string
  • append() – 向 tag 中追加内容
  • extend() – 4.7.0 新增方法，扩展 tag 中的内容
  • NavigableString() & .new_tag() – 向 tag 中添加新文本或新标签
  • insert() – 向 tag 中插入内容，可设定插入位置
  • insert_before() & insert_after() – 在当前 tag 前 (或后) 插入内容
  • clear() – 清理当前 tag 中的内容
  • extract() – 从文档树中移除当前 tag，并返回被移除的 tag
  • decompose() – 从文档树中移除当前 tag，并完全销毁
  • replace_with() – 替换文档树中的内容
  • wrap() – 打包指定元素
  • unwrap() – 解包指定元素

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2019-07-17
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