BeautifulSoup 按复合类名搜索时返回空列表 [英] BeautifulSoup returns empty list when searching by compound class names

问题描述

使用正则表达式按复合类名称搜索时，BeautifulSoup 返回空列表.

BeautifulSoup returns empty list when searching by compound class names using regex.

示例:

import re
from bs4 import BeautifulSoup

bs = 
    """
    <a class="name-single name692" href="www.example.com"">Example Text</a>
    """

bsObj = BeautifulSoup(bs)

# this returns the class
found_elements = bsObj.find_all("a", class_= re.compile("^(name-single.*)$"))

# this returns an empty list
found_elements = bsObj.find_all("a", class_= re.compile("^(name-single named*)$"))

我需要非常精确的类选择.有什么想法吗?

I need the class selection to be very precise. Any ideas?

推荐答案

不幸的是，当您尝试对包含多个类的类属性值进行正则表达式匹配时，BeautifulSoup 会应用正则表达式分别表达到每个类.以下是有关该问题的相关主题:

Unfortunately, when you try to make a regular expression match on a class attribute value that contains multiple classes, BeautifulSoup would apply the regular expression to every single class separately. Here are the relevant topics about the problem:

• Beautiful Soup 的 Python 正则表达式
• 多CSS类搜索不方便

这都是因为class 是一个非常特殊的多值属性，每次解析 HTML 时，BeautifulSoup 的树构建器之一(取决于解析器选择)在内部拆分类字符串value 到一个类列表中(引用自 HTMLTreeBuilder 的文档字符串):

This is all because class is a very special multi-valued attribute and every time you parse HTML, one of the BeautifulSoup's tree builders (depending on the parser choice) internally splits a class string value into a list of classes (quote from the HTMLTreeBuilder's docstring):

# The HTML standard defines these attributes as containing a
# space-separated list of values, not a single value. That is,
# class="foo bar" means that the 'class' attribute has two values,
# 'foo' and 'bar', not the single value 'foo bar'.  When we
# encounter one of these attributes, we will parse its value into
# a list of values if possible. Upon output, the list will be
# converted back into a string.

<小时>

有多种变通方法，但这里有一个 hack-ish - 我们将要求 BeautifulSoup 不要将 class 作为多值属性处理简单的自定义树生成器:

---

There are multiple workarounds, but here is a hack-ish one - we are going to ask BeautifulSoup not to handle class as a multi-valued attribute by making our simple custom tree builder:

import re

from bs4 import BeautifulSoup
from bs4.builder._htmlparser import HTMLParserTreeBuilder


class MyBuilder(HTMLParserTreeBuilder):
    def __init__(self):
        super(MyBuilder, self).__init__()

        # BeautifulSoup, please don't treat "class" specially
        self.cdata_list_attributes["*"].remove("class")


bs = """<a class="name-single name692" href="www.example.com"">Example Text</a>"""
bsObj = BeautifulSoup(bs, "html.parser", builder=MyBuilder())
found_elements = bsObj.find_all("a", class_=re.compile(r"^name-single named+$"))

print(found_elements)

在这种情况下，正则表达式将作为一个整体应用于 class 属性值.

In this case the regular expression would be applied to a class attribute value as a whole.

或者，您可以在启用 xml 功能的情况下解析 HTML(如果适用):

Alternatively, you can just parse the HTML with xml features enabled (if this is applicable):

soup = BeautifulSoup(data, "xml")

<小时>

您还可以使用 CSS 选择器 并匹配具有 name-single 类和以name"开头的类的所有元素:

---

You can also use CSS selectors and match all elements with name-single class and a class staring with "name":

soup.select("a.name-single,a[class^=name]")

然后您可以根据需要手动应用正则表达式:

You can then apply the regular expression manually if needed:

pattern = re.compile(r"^name-single named+$")
for elm in bsObj.select("a.name-single,a[class^=name]"):
    match = pattern.match(" ".join(elm["class"]))
    if match:
        print(elm)

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