Don't copy styles only to change margins, borders and paddings

[liZe]

Many situations force to override (often remove) margins, borders and paddings, including:

• table cells with collapsing borders,
• blocks split between pages,
• inline boxes split by block boxes.

Depending on the situation, the style is copied and modified, or the computed values are applied to the box (box.padding_right and its friends). There's no perfect solution, as the first one greatly increases memory usage, and the second one breaks min/max-width calculation and box copy.

We have to find a solution to:

• never copy nor modify style dicts,
• override margins, borders and paddings on styles without modifying the dict,
• avoid mixing box.style['margin_left'] and box.margin_left (as it's done in mix/max-width calculation on boxes after dimensions are already set on the box),
• keep speed (i.e. don't use the copy-on-write method used before, as it was overriding the __getitem__ method that's over-optimized, see 9abc034).

[liZe]

In this wonderful blog entry, the solution I was thinking of is given in the chapter called "A sidenote: style struct sharing".

[liZe]

4057a92 uses real dicts for performance and use ChainMaps to keep original dicts when changing padding/borders/margins, breaking the Python2 compatibility.

[liZe]

See #948 (comment) for more information about where to remove style edition. #950 removed the worse parts, but it’s not finished yet.

Having frozen styles with computed values is the way to go.

[Tontyna]

Having frozen styles with computed values is the way to go.

Of course. But grepping for modifications of Box.style it looks like since #950 we don't touch the immutable computed values anymore. The modfications rather ensure the correct initialization of computed values for specialized boxes.

What I don't like is the fact that the pagination stores used values (and other properties related to layout) as attributes in the root_box tree and its children. AFAIK it doesn't hurt, not sure whether it's required, but it definitely feels wrong.

A 'style struct sharing' would be great. At the moment each box of the formatted root_box tree has its own complete style dictionary -- style sharing only happens between the formatted box and its paginated counterpart(s).