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Beautiful Soup Differences

Soup Sieve is the official CSS "select" implementation of Beautiful Soup 4.7.0+. While the inclusion of Soup Sieve fixes many issues and greatly expands CSS support in Beautiful Soup, it does introduce some differences which may surprise some who've become accustom to the old "select" implementation.

Beautiful Soup's old select method had numerous limitations and quirks that do not align with the actual CSS specifications. Most are insignificant, but there are a couple differences that people over the years had come to rely on. Soup Sieve, which aims to follow the CSS specification closely, does not support these differences.

Attribute Values

Beautiful Soup was very relaxed when it came to attribute values in selectors: [attribute=value]. Beautiful Soup would allow almost anything for a valid unquoted value. Soup Sieve, on the other hand, follows the CSS specification and requires that a value be a valid identifier, or it must be quoted. If you get an error complaining about a malformed attribute, you may need to quote the value.

For instance, if you previously used a selector like this:

soup.select('[div={}]')

You would need to quote the value as {} is not a valid CSS identifier, so it must be quoted:

soup.select('[div="{}"]')

CSS Identifiers

Since Soup Sieve follows the CSS specification, class names, id names, tag names, etc. must be valid identifiers. Since identifiers, according to the CSS specification, cannot start with a number, some users may find that their old class, id, or tag name selectors that started with numbers will not work. To specify such selectors, you'll have to use CSS escapes.

So if you used to use:

soup.select('.2class')

You would need to update with:

soup.select(r'.\32 class')

Numbers in the middle or at the end of a class will work as they always did:

soup.select('.class2')

Relative Selectors

Whether on purpose or on accident, Beautiful Soup used to allow relative selectors:

soup.select('> div')

The above is not a valid CSS selector according the CSS specifications. Relative selector lists have only recently been added to the CSS specifications, and they are only allowed in a :has() pseudo-class:

article:has(> div)

But, in the level 4 CSS specifications, the :scope pseudo-class has been added which allows for the same feel as using > div. Since Soup Sieve supports the :scope pseudo-class, it can be used to produce the same behavior as the legacy select method.

In CSS, the :scope pseudo-class represents the element that the CSS select operation is called on. In supported browsers, the following JavaScript example would treats :scope as the element that el references:

el.querySelectorAll(':scope > .class')

Just like in the JavaScript example above, Soup Sieve would also treat :scope as the element that el references:

el.select(':scope > .class')

In the case where the element is the document node, :scope would simply represent the root element of the document.

So, if you used to to have selectors such as:

soup.select('> div')

You can simply add :scope, and it should work the same:

soup.select(':scope > div')

While this will generally give you what is expected for the relative, descendant selectors, this will not work for sibling selectors, and the reasons why are covered in more details in Out of Scope Selectors.

Out of Scope Selectors

In a browser, when requesting a selector via querySelectorAll, the element that querySelectorAll is called on is the scoped element. So in the following example, el is the scoped element.

el.querySelectorAll('.class')

This same concept applies to Soup Sieve, where the element that select or select_one is called on is also the scoped element. So in the following example, el is also the scoped element:

el.select('.class')

In browsers, querySelectorAll and querySelector only return elements under the scoped element. They do not return the scoped element itself, its parents, or its siblings. Only when querySelectorAll or querySelector is called on the document node will it return the scoped selector, which would be the root element, as the query is being called on the document itself and not the scoped element.

Soup Sieve aims to essentially mimic the browser functions such as querySelector, querySelectorAll, matches, etc. In Soup Sieve select and select_one are analogous to querySelectorAll and querySelector respectively. For this reason, Soup Sieve also only returns elements under the scoped element. The idea is to provide a familiar interface that behaves, as close as possible, to what people familiar with CSS selectors are used to.

So while Soup Sieve will find elements relative to :scope with > or  :

soup.select(':scope > div')

It will not find elements relative to :scope with + or ~ as siblings to the scoped element are not under the scoped element:

soup.select(':scope + div')

This is by design and is in align with the behavior exhibited in all web browsers.

Selected Element Order

Another quirk of Beautiful Soup's old implementation was that it returned the HTML nodes in the order of how the selectors were defined. For instance, Beautiful Soup, if given the pattern article, body would first return <article> and then <body>.

Soup Sieve does not, and frankly cannot, honor Beautiful Soup's old ordering convention due to the way it is designed. Soup Sieve returns the nodes in the order they are defined in the document as that is how the elements are searched. This much more efficient and provides better performance.

So, given the earlier selector pattern of article, body, Soup Sieve would return the element <body> and then <article> as that is how it is ordered in the HTML document.