MEP26: Artist styling#

Abstract #

This MEP proposes a new stylesheet implementation to allow more comprehensive and dynamic styling of artists.

The current version of matplotlib (1.4.0) allows stylesheets based on the rcParams syntax to be applied before creation of a plot. The methodology below proposes a new syntax, based on CSS, which would allow styling of individual artists and properties, which can be applied dynamically to existing objects.

This is related to (and makes steps toward) the overall goal of moving to a DOM/tree-like architecture.

Detailed description #

Currently, the look and appearance of existing artist objects (figure, axes, Line2D, etc.) can only be updated via set_ and get_ methods on the artist object, which is quite laborious, especially if no reference to the artist(s) has been stored. The new style sheets introduced in 1.4 allow styling before a plot is created, but do not offer any means to dynamically update plots or distinguish between artists of the same type (i.e. to specify the line color and line style separately for differing Line2D objects).

The initial development should concentrate on allowing styling of artist primitives (those Artists that do not contain other Artists), and further development could expand the CSS syntax rules and parser to allow more complex styling. See the appendix for a list of primitives.

The new methodology would require development of a number of steps:

A new stylesheet syntax (likely based on CSS) to allow selection of artists by type, class, id, etc.
A mechanism by which to parse a stylesheet into a tree
A mechanism by which to translate the parse-tree into something which can be used to update the properties of relevant artists. Ideally this would implement a method by which to traverse the artists in a tree-like structure.
A mechanism by which to generate a stylesheet from existing artist properties. This would be useful to allow a user to export a stylesheet from an existing figure (where the appearance may have been set using the matplotlib API)...

Implementation #

It will be easiest to allow a '3rd party' to modify/set the style of an artist if the 'style' is created as a separate class and store against the artist as a property. The GraphicsContextBase class already provides a the basis of a Style class and an artist's draw method can be refactored to use the Style class rather than setting up its own GraphicsContextBase and transferring its style-related properties to it. A minimal example of how this could be implemented is shown here: https://github.com/JamesRamm/mpl_experiment

IMO, this will also make the API and code base much neater as individual get/set methods for artist style properties are now redundant... Indirectly related would be a general drive to replace get/set methods with properties. Implementing the style class with properties would be a big stride toward this...

For initial development, I suggest developing a syntax based on a much (much much) simplified version of CSS. I am in favour of dubbing this Artist Style Sheets :+1: :

BNF Grammar #

I propose a very simple syntax to implement initially (like a proof of concept), which can be expanded upon in the future. The BNF form of the syntax is given below and then explained

RuleSet ::= SelectorSequence "{"Declaration"}"

SelectorSequence :: = Selector {"," Selector}

Declaration ::= propName":" propValue";"

Selector ::= ArtistIdent{"#"Ident}

propName ::= Ident

propValue ::= Ident | Number | Colour | "None"

ArtistIdent, Ident, Number and Colour are tokens (the basic building blocks of the expression) which are defined by regular expressions.

Syntax #

A CSS stylesheet consists of a series of rule sets in hierarchical order (rules are applied from top to bottom). Each rule follows the syntax

selector {attribute: value;}

Each rule can have any number of attribute: value pairs, and a stylesheet can have any number of rules.

The initial syntax is designed only for Artist primitives. It does not address the question of how to set properties on Container types (whose properties may themselves be Artists with settable properties), however, a future solution to this could simply be nested RuleSets

Selectors #

Selectors define the object to which the attribute updates should be applied. As a starting point, I propose just 2 selectors to use in initial development:

Artist Type Selector

Select an Artist by it's type. E.g Line2D or Text:

Line2D {attribute: value}

The regex for matching the artist type selector (ArtistIdent in the BNF grammar) would be:

ArtistIdent = r'(?P<ArtistIdent>\bLine2D\b|\bText\b|\bAxesImage\b|\bFigureImage\b|\bPatch\b)'

GID selector #

Select an Artist by its gid:

Line2D#myGID {attribute: value}

A gid can be any string, so the regex could be as follows:

Ident = r'(?P<Ident>[a-zA-Z_][a-zA-Z_0-9]*)'

The above selectors roughly correspond to their CSS counterparts (http://www.w3.org/TR/CSS21/selector.html)

Attributes and values #

Attributes are any valid (settable) property for the Artist in question.
Values are any valid value for the property (Usually a string, or number).

Parsing #

Parsing would consist of breaking the stylesheet into tokens (the python cookbook gives a nice tokenizing recipe on page 66), applying the syntax rules and constructing a Tree. This requires defining the grammar of the stylesheet (again, we can borrow from CSS) and writing a parser. Happily, there is a recipe for this in the python cookbook as well.

Visitor pattern for matplotlib figure #

In order to apply the stylesheet rules to the relevant artists, we need to 'visit' each artist in a figure and apply the relevant rule. Here is a visitor class (again, thanks to python cookbook), where each node would be an artist in the figure. A visit_ method would need to be implemented for each mpl artist, to handle the different properties for each

class Visitor:
    def visit(self, node):
       name = 'visit_' + type(node).__name__
       meth = getattr(self, name, None)
       if meth is None:
          raise NotImplementedError
       return meth(node)

An evaluator class would then take the stylesheet rules and implement the visitor on each one of them.

Backward compatibility #

Implementing a separate Style class would break backward compatibility as many get/set methods on an artist would become redundant. While it would be possible to alter these methods to hook into the Style class (stored as a property against the artist), I would be in favor of simply removing them to both neaten/simplify the codebase and to provide a simple, uncluttered API...

Line2D
Text
AxesImage
FigureImage
Patch