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Soup Sieve implements most of the selectors from the stable specification and even many from the latest draft specification. Selectors can be used to detect and filter elements. To learn more about which specific selectors are implemented, see CSS Selectors.

Soup Sieve will detect the document type being used from the Beautiful Soup object that is given to it, and depending on the document type, its behavior may be slightly different.

When detecting XHTML, Soup Sieve simply looks to see if the root element of an XML document is under the XHTML namespace and does not currently look at the doctype. If in the future there is a need for stricter XHTML detection, this may change.

  • HTML document types (HTML, HTML5) will have their tag names and attribute names treated without case sensitivity, like most browsers do.

  • XML document types (including XHTML) will have their tag names and attribute names treated with case sensitivity.

  • HTML5, XHTML and XML documents will have namespaces evaluated per the document's support (provided via the parser). Some additional configuration is required when using namespaces, see Namespace for more information.

    Getting Proper Namespaces

    The html5lib parser provides proper namespaces for HTML5, but lxml's HTML parser will not. If you need namespace support for HTML5, consider using html5lib.

    For XML, the lxml-xml parser (xml for short) will provide proper namespaces. It is generally suggested that lxml-xml is used to parse XHTML documents to take advantage of namespaces.

  • While attribute values are generally treated as case sensitive, HTML5 and HTML treat the type attribute special. The type attribute's value is always case insensitive. This is generally how most browsers treat type. If you need type to be sensitive, you can use the s flag: [type="submit" s].

While Soup Sieve access is exposed through Beautiful Soup's API, Soup Sieve's API can always be imported and accessed directly for more controlled tag selection if needed.



Print debug output when parsing a selector.

>>> import soupsieve as sv
>>> sv.compile('p:has(#id) > span.some-class:contains(text)', flags=sv.DEBUG)
## PARSING: 'p:has(#id) > span.some-class:contains(text)'
TOKEN: 'tag' --> 'p' at position 0
TOKEN: 'pseudo_class' --> ':has(' at position 1
    is_pseudo: True
    is_open: True
    is_relative: True
TOKEN: 'id' --> '#id' at position 6
TOKEN: 'pseudo_close' --> ')' at position 9
TOKEN: 'combine' --> ' > ' at position 10
TOKEN: 'tag' --> 'span' at position 13
TOKEN: 'class' --> '.some-class' at position 17
TOKEN: 'pseudo_contains' --> ':contains(text)' at position 28
SoupSieve(pattern='p:has(#id) > span.some-class:contains(text)', namespaces=None, custom=None, flags=1)


def select_one(select, tag, namespaces=None, flags=0, **kwargs):
    """Select the specified tags."""

select_one will return the first tag under the given tag that matches the given CSS selectors provided, or it will return None if a suitable tag was not found.

select_one accepts a CSS selector string, a Tag/BeautifulSoup object, an optional namespace dictionary, and flags.

>>> import soupsieve as sv
>>> sv.select_one('p:is(.a, .b, .c)', soup)
<p class="a">Cat</p>

def select(select, tag, namespaces=None, limit=0, flags=0, **kwargs):
    """Select the specified tags."""

select will return all tags under the given tag that match the given CSS selectors provided. You can also limit the number of tags returned by providing a positive integer via the limit parameter (0 means to return all tags).

select accepts a CSS selector string, a Tag/BeautifulSoup object, an optional namespace dictionary, a limit, and flags.

>>> import soupsieve as sv
>>>'p:is(.a, .b, .c)', soup)
[<p class="a">Cat</p>, <p class="b">Dog</p>, <p class="c">Mouse</p>]

def iselect(select, node, namespaces=None, limit=0, flags=0, **kwargs):
    """Select the specified tags."""

iselect is exactly like select except that it returns a generator instead of a list.


def closest(select, tag, namespaces=None, flags=0, **kwargs):
    """Match closest ancestor to the provided tag."""

closest returns the tag closest to the given tag that matches the given selector. The element found must be a direct ancestor of the tag or the tag itself.

closest accepts a CSS selector string, a Tag/BeautifulSoup object, an optional namespace dictionary, and flags.


def match(select, tag, namespaces=None, flags=0, **kwargs):
    """Match node."""

The match function matches a given tag with a given CSS selector.

match accepts a CSS selector string, a Tag/BeautifulSoup object, an optional namespace dictionary, and flags.

>>> nodes ='p:is(.a, .b, .c)', soup)
>>> sv.match('p:not(.b)', nodes[0])
>>> sv.match('p:not(.b)', nodes[1])


def filter(select, nodes, namespaces=None, flags=0, **kwargs):
    """Filter list of nodes."""

filter takes an iterable containing HTML nodes and will filter them based on the provided CSS selector string. If given a Tag/BeautifulSoup object, it will iterate the direct children filtering them.

filter accepts a CSS selector string, an iterable containing nodes, an optional namespace dictionary, and flags.

>>> sv.filter('p:not(.b)', soup.div)
[<p class="a">Cat</p>, <p class="c">Mouse</p>]


def escape(ident):
    """Escape CSS identifier."""

escape is used to escape CSS identifiers. It follows the CSS specification and escapes any character that would normally cause an identifier to be invalid.

>>> sv.escape(".foo#bar")
>>> sv.escape("()[]{}")
>>> sv.escape('--a')
>>> sv.escape('0')
'\\30 '
>>> sv.escape('\0')

New in 1.9.0

escape is a new API function added in 1.9.0.


def compile(pattern, namespaces=None, flags=0, **kwargs):
    """Compile CSS pattern."""

compile will pre-compile a CSS selector pattern returning a SoupSieve object. The SoupSieve object has the same selector functions available via the module without the need to specify the selector, namespaces, or flags.

class SoupSieve:
    """Match tags in Beautiful Soup with CSS selectors."""

    def match(self, tag):

    def closest(self, tag):
        """Match closest ancestor."""

    def filter(self, iterable):

    def select_one(self, tag):
        """Select a single tag."""

    def select(self, tag, limit=0):
        """Select the specified tags."""

    def iselect(self, tag, limit=0):
        """Iterate the specified tags."""


Soup Sieve caches compiled patterns for performance. If for whatever reason, you need to purge the cache, simply call purge.

Custom Selectors

The custom selector feature is loosely inspired by the css-extensions proposal. In its current form, Soup Sieve allows assigning a complex selector to a custom pseudo-class name. The pseudo-class name must start with :-- to avoid conflicts with any future pseudo-classes.

To create custom selectors, you simply need to pass a dictionary containing the custom pseudo-class names (keys) with the associated CSS selectors that the pseudo-classes are meant to represent (values). It is important to remember that pseudo-class names are not case sensitive, so even though a dictionary will allow you to specify multiple keys with the same name (as long as the character cases are different), Soup Sieve will not and will throw an exception if you attempt to do so.

In the following example, we will define our own custom selector called :--header that will be an alias for h1, h2, h3, h4, h5, h6.

import soupsieve as sv
import bs4

markup = """
<h1 id="1">Header 1</h1>
<h2 id="2">Header 2</h2>
<p id="3"></p>
<p id="4"><span>child</span></p>

soup = bs4.BeautifulSoup(markup, 'lxml')
print(':--header', soup, custom={':--header': 'h1, h2, h3, h4, h5, h6'}))

The above code, when run, should yield the following output:

[<h1 id="1">Header 1</h1>, <h2 id="2">Header 2</h2>]

Custom selectors can also be dependent upon other custom selectors. You don't have to worry about the order in the dictionary as custom selectors will be compiled "just in time" when they are needed. Be careful though, if you create a circular dependency, you will get a SelectorSyntaxError.

Assuming the same markup as in the first example, we will now create a custom selector that should find any element that has child elements, we will call the selector :--parent. Then we will create another selector called :--parent-paragraph that will use the :--parent selector to find <p> elements that are also parents:

custom = {
    ":--parent": ":has(> *|*)",
    ":--parent-paragraph": "p:--parent"
print(':--parent-paragraph', soup, custom=custom))

The above code will yield the only paragraph that is a parent:

[<p id="4"><span>child</span></p>]


Many of Soup Sieve's selector functions take an optional namespace dictionary. Namespaces, just like CSS, must be defined for Soup Sieve to evaluate ns|tag type selectors. This is analogous to CSS's namespace at-rule:

@namespace url("");
@namespace svg url("");

A namespace dictionary should have keys (prefixes) and values (namespaces). An empty key string for a key would denote the default key. An empty value would essentially represent a null namespace. To represent the above CSS example for Soup Sieve, we would configure it like so:

namespace = {
    "": "",   # Default namespace is for XHTML
    "svg": "",  # The SVG namespace defined with prefix of "svg"

Prefixes used in the namespace dictionary do not have to match the prefixes in the document. The provided prefix is never compared against the prefixes in the document, only the namespaces are compared. The prefixes in the document are only there for the parser to know which tags get which namespace. And the prefixes in the namespace dictionary are only defined in order to provide an alias for the namespaces when using the namespace selector syntax: ns|name.

Tags do not necessarily have to have a prefix for Soup Sieve to recognize them either. For instance, in HTML5, SVG should automatically get the SVG namespace. Depending how namespaces were defined in the document, tags may inherit namespaces in some conditions. Namespace assignment is mainly handled by the parser and exposed through the Beautiful Soup API. Soup Sieve uses the Beautiful Soup API to then compare namespaces for supported documents.

Last update: August 31, 2023