Truncate HTML to a length, keeping tags balanced

Cutting an HTML string at a character count with a slice breaks it: you can land in the middle of a tag or leave an element open. Truncating the tree instead keeps the markup well-formed. Parse the HTML, walk it counting visible text, and drop everything past the budget; because you edit nodes rather than a string, every tag you keep stays balanced.

The walk trims the text node that crosses the limit and removes whole subtrees after it with decompose():

import turbohtml
from turbohtml import Element, Text


def truncate(html, limit):
    body = turbohtml.parse(html).find("body")
    _trim(body, limit)
    return "".join(child.serialize() for child in body)


def _trim(node, budget):
    for child in list(node):
        if budget <= 0:
            child.decompose()
        elif isinstance(child, Text):
            if len(child.data) > budget:
                child.data = child.data[:budget].rstrip() + "…"
                budget = 0
            else:
                budget -= len(child.data)
        elif isinstance(child, Element):
            budget = _trim(child, budget)
    return budget


print(truncate("<div><p>Hello world, this is long.</p><p>Second para.</p></div>", 11))

<div><p>Hello world…</p></div>

The count is over visible text, so tags do not eat the budget, and the second paragraph – reached after the budget ran out – drops whole rather than emptied. An element trimmed mid-text keeps its wrapper:

print(truncate("<article><h2>Title</h2><p>Body text that is fairly long here.</p></article>", 9))

<article><h2>Title</h2><p>Body…</p></article>

This is the Django Truncator.chars and TruncateHTMLParser behavior without the parser subclass: assign a Text.data to rewrite a run of text, and call decompose() to remove a node and its subtree. To keep a whole region rather than a length, Trim a document to a selector trims to a selector instead.