A (partial) implementation of the CSS box and flexbox layout algorithm.

The following CSS attributes and value types are supported:

Name Value
width, height positive number
min_width, min_height positive number
max_width, max_height positive number
left, right, top, bottom number
margin, margin_left, margin_right, margin_top, margin_bottom number
padding, padding_left, padding_right, padding_top, padding_bottom positive number
border_width, border_left_width, border_right_width, border_top_width, border_bottom_width positive number
flex_direction "column", "row"
justify_content "flex-start", "center", "flex-end", "space-between", "space-around"
align_items, align_self "flex-start", "center", "flex-end", "stretch"
flex positive number
flex_wrap "wrap", "nowrap"
position "relative", "absolute"


In your virtualenv, install Colosseum:

$ pip install colosseum

Then, you can instantiate ``CSSnode``s, and query the layout that results:

>>> from colosseum import CSSNode, ROW, COLUMN
>>> node = CSSNode(width=1000, height=1000, flex_direction=ROW)
>>> node.children.add(CSSNode(width=100, height=200))
>>> node.children.add(CSSNode(width=300, height=150))
>>> layout = node.layout
>>> print(layout)
<Layout (1000x1000 @ 0,0)>
>>> layout.width
>>> layout.height
>>> layout.left
>>> for child in node.children:
...     print(child.layout)
<Layout (100x200 @ 0,0)>
<Layout (300x150 @ 100,0)>

Requesting the layout attribute of a CSSNode forces the box model to be evaluated. Once evaluated, the layout will be cached. Modifying any CSS property on a node will mark the layout as dirty, and the layout will be recomputed the next time the layout is accessed. For example, if we switch the outer node to be a “column” flex box, rather than a “row” flex box, you’ll see the coordinates of the child boxes update to reflect a vertical, rather than horizontal layout:

>>> node.flex_direction = COLUMN
>>> print(node.layout)
<Layout (1000x1000 @ 0,0)>
>>> for child in node.children:
...     print(child.layout)
<Layout (100x200 @ 0,0)>
<Layout (300x150 @ 0,200)>

Style attributes can also be set in bulk, using the style() method on a CSSNode:

>>>, height=800)
>>> print(node.layout)
<Layout (1500x800 @ 0,0)>

Style attributes can also be removed by deleting the attribute on the CSSNode. The value of the property will revert to the default:

>>>, margin_left=20)
>>> print(node.layout)
<Layout (1500x800 @ 20,10)>
>>> del(node.margin_left)
>>> print(node.margin_left)
>>> print(node.layout)
<Layout (1500x800 @ 0,10)>

Child nodes can also be defined declaratively by providing the child nodes as arguments to the parent node at time of construction. The original example could have been defined as follows:

>>> node = CSSNode(
...     CSSNode(width=100, height=200),
...     CSSNode(width=300, height=150),
...     width=1000,
...     height=1000,
...     flex_direction=ROW
... )


Colosseum is part of the BeeWare suite. You can talk to the community through:


If you experience problems with Colosseum, log them on GitHub. If you want to contribute code, please fork the code and submit a pull request.


The algorithm and test suite for this library is a language port of CSS-layout project, open-sourced by Facebook.