How to Work with the Layout Engine in Python

The layout layer of Aspose.HTML FOSS for Python computes geometry from styled documents: build_box_tree() derives the box structure, layout_block_tree() places block fragments, and paginate_fragments() splits output into pages. This guide prepares a styled document — the required input — and explains each layout stage, using pip for installation.

Step-by-Step Guide

Step 1: Install the Package

pip install aspose-html-foss

Step 2: Import Required Classes

from aspose_html.dom import Document
from aspose_html.cssom import CSSStyleSheet

Step 3: Prepare a Styled Document

Layout consumes computed styles, so build and style the tree first:

doc = Document()
el = doc.create_element("div")
doc.append_child(el)
sheet = CSSStyleSheet()
sheet.replace_sync("div { color: red }")
doc.attach_style_sheet(sheet)
print(el.get_computed_style().get_property_value("color"))

Step 4: Derive the Box Tree

build_box_tree() produces a BoxRoot of BoxNode entries, classifying each element’s display model with classify_display() against the Display enumeration; ComputedStyle and EdgeSizes carry per-box style and box-model measurements.

Step 5: Run Layout and Paginate

layout_block_tree() yields a FragmentRoot of BlockFragment geometry; inline content assembles into LineFragment lines via layout_inline_fragments(); paginate_fragments() splits results into PageFragment pages honouring BreakHints.

Common Issues and Fixes

Layout output is empty. The input tree had no styled content — attach stylesheets and content before layout.

A box has an unexpected display classification. Check classify_display() for the element; the computed display value drives it.

Page breaks land in odd places. Provide BreakHints to paginate_fragments() to guide break opportunities.

Frequently Asked Questions

What is the difference between boxes and fragments?

Boxes (BoxNode) describe layout structure derived from styles; fragments (BlockFragment, LineFragment, PageFragment) are placed results with geometry.

Where do margins and padding resolve?

In EdgeSizes on each box — the first place to look when dimensions surprise you.

Does the engine shape text?

Inline content is shaped into ShapedRun pieces before line assembly.