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问题:

The Question:

BeautifulSoup provides a very limited support for CSS selectors. For instance, the only supported pseudo-class is nth-of-type and it can only accept numerical values - arguments like even or odd are not allowed.

Is it possible to extend BeautifulSoup CSS selectors or let it use lxml.cssselect internally as an underlying CSS selection mechanism?

Let's take a look at an example problem/use case. Locate only even rows in the following HTML:

<table>
    <tr>
        <td>1</td>
    <tr>
        <td>2</td>
    </tr>
    <tr>
        <td>3</td>
    </tr>
    <tr>
        <td>4</td>
    </tr>
</table>

In lxml.html and lxml.cssselect, it is easy to do via :nth-of-type(even):

from lxml.html import fromstring
from lxml.cssselect import CSSSelector

tree = fromstring(data)

sel = CSSSelector('tr:nth-of-type(even)')

print [e.text_content().strip() for e in sel(tree)]

But, in BeautifulSoup:

print(soup.select("tr:nth-of-type(even)"))

would throw an error:

NotImplementedError: Only numeric values are currently supported for the nth-of-type pseudo-class.

Note that we can workaround it with .find_all():

print([row.get_text(strip=True) for index, row in enumerate(soup.find_all("tr"), start=1) if index % 2 == 0])

回答1:

After checking the source code, it seems that BeautifulSoup does not provide any convenient point in its interface to extend or monkey patch its existing functionality in this regard. Using functionality from lxml is not possible either since BeautifulSoup only uses lxml during parsing and uses the parsing results to create its own respective objects from them. The lxml objects are not preserved and cannot be accessed later.

That being said, with enough determination and with the flexibility and introspection capabilities of Python, anything is possible. You can modify the BeautifulSoup method internals even at run-time:

import inspect
import re
import textwrap

import bs4.element


def replace_code_lines(source, start_token, end_token,
                       replacement, escape_tokens=True):
    """Replace the source code between `start_token` and `end_token`
    in `source` with `replacement`. The `start_token` portion is included
    in the replaced code. If `escape_tokens` is True (default),
    escape the tokens to avoid them being treated as a regular expression."""

    if escape_tokens:
        start_token = re.escape(start_token)
        end_token = re.escape(end_token)

    def replace_with_indent(match):
        indent = match.group(1)
        return textwrap.indent(replacement, indent)

    return re.sub(r"^(\s+)({}[\s\S]+?)(?=^\1{})".format(start_token, end_token),
                  replace_with_indent, source, flags=re.MULTILINE)


# Get the source code of the Tag.select() method
src = textwrap.dedent(inspect.getsource(bs4.element.Tag.select))

# Replace the relevant part of the method
start_token = "if pseudo_type == 'nth-of-type':"
end_token = "else"
replacement = """\
if pseudo_type == 'nth-of-type':
    try:
        if pseudo_value in ("even", "odd"):
            pass
        else:
            pseudo_value = int(pseudo_value)
    except:
        raise NotImplementedError(
            'Only numeric values, "even" and "odd" are currently '
            'supported for the nth-of-type pseudo-class.')
    if isinstance(pseudo_value, int) and pseudo_value < 1:
        raise ValueError(
            'nth-of-type pseudo-class value must be at least 1.')
    class Counter(object):
        def __init__(self, destination):
            self.count = 0
            self.destination = destination

        def nth_child_of_type(self, tag):
            self.count += 1
            if pseudo_value == "even":
                return not bool(self.count % 2)
            elif pseudo_value == "odd":
                return bool(self.count % 2)
            elif self.count == self.destination:
                return True
            elif self.count > self.destination:
                # Stop the generator that's sending us
                # these things.
                raise StopIteration()
            return False
    checker = Counter(pseudo_value).nth_child_of_type
"""
new_src = replace_code_lines(src, start_token, end_token, replacement)

# Compile it and execute it in the target module's namespace
exec(new_src, bs4.element.__dict__)
# Monkey patch the target method
bs4.element.Tag.select = bs4.element.select

This is the portion of code being modified.

Of course, this is everything but elegant and reliable. I don't envision this being seriously used anywhere, ever.