Colosseum

An independent implementation of the CSS layout algorithm. This implementation is completely standalone - it isn't dependent on a browser, and can be run over any box-like set of objects that need to be laid out on a page (either physical or virtual)

At present, the implementation is partial; only portions of the box and flexbox section of the specification are defined:

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"

Quickstart

In your virtualenv, install Colosseum:

$ pip install colosseum

Colosseum provides a CSS class that allows you to define CSS properties, and apply them can be applied to any DOM-like tree of objects. There is no required base class; Colosseum will duck-type any object providing the required API. The simplest possible DOM node is the following:

class MyDOMNode:
    def __init__(self, style):
        self.parent = None
        self.children = []
        self.style = style.apply(self)

    def add(self, child):
        self.children.append(child)
        child.parent = self

That is, a node must provide:

• a parent attribute, declaring the parent in the DOM tree; the root of the DOM tree has a parent of None.
• a children attribute, containing the list of DOM child nodes.
• a style attribute, generated by calling apply() on a CSS declaration.

With that a compliant DOM node definition, you can then and query the layout that results:

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

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

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

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

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

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

Layout values are given relative to their parent node. If you want to know the absolute position of a node on the display canvas, you can request the origin attribute of the layout. This will give you the point on the canvas from which all the node's attributes are measured. You can also request the absolute attribute of the layout, which will give you the position of the element on the entire canvas:

>>> node.style.set(margin_top=10, margin_left=20)
>>> print(node.style.layout)
<Layout (1500x800 @ 20,10)>
>>> for child in node.children:
...     print(child.style.layout)
<Layout (100x200 @ 0,0)>
<Layout (300x150 @ 0,200)>
>>> print(node.style.layout.origin)
<Point (0,0)>
>>> for child in node.children:
...     print(child.style.layout.origin)
<Point (20,10)>
<Point (20,10)>
>>> print(node.style.layout.absolute)
<Point (20,10)>
>>> for child in node.children:
...     print(child.style.layout.absolute)
<Point (20,10)>
<Point (20,210)>

Community

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

• @pybeeware on Twitter
• The pybee/general channel on Gitter.

We foster a welcoming and respectful community as described in our BeeWare Community Code of Conduct.

Contributing

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.

Acknowledgements

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