I love programming with Qt4, so here are two considerations concerning layout handling based on that toolkit.

Graphical user interfaces usually consist of a bunch of widgets that are hierarchically organized by layout managers. The basic layout classes implement Vertical, Horizontal and Grid alignment. The widgets include attributes like spacing, padding or scaling policies the layout managers can take into account. Although this works perfectly for applications this model probably isn't flexible enough for plotting figures. For example, there is no layout manager equivalent for a legend being placed on top of a axes widget and being aligned to some edge.

I don't know if you have had the chance of looking into QML, but I think this way of aligning objects could be a really powerful solution! Every object has anchor attributes like top, bottom, left, right, center. Aligning two objects is as simple as defining an expression object1.anchors.left = object2.anchors.right. I also think that it would be awesome if lengths/coordinates could be defined as unit-aware objects, so a position/distance/length could be given in pixels, data coordinates, inches, centimeters, font-height etc. This way, the upper-right plot legend, spaced by 1.5 times the height of 'M', could be defined like

legend.anchors.top = axes.anchors.top - length(1.5, "em")
legend.anchors.right = axes.anchors.right - length(1.5, "em")

QML interprets these statements as bindings that are dynamically evaluated on demand, so the python equivalent of the right hand side would be a lambda function. Such alignments determine the position, width and height of an object unless the object defines a fixed value for one or multiple of these attributes.

Solving the dependencies of multiple objects referencing each other is a little bit tricky.. but I think I already got the python code for that task! In one of my projects I implemented a data table where a user can define an equation for a table cell that may depend on the values of other cells, just like Excel :). Using the python ast-module I figure out which objects are referenced in an expression and build a dependency tree. This tree is then solved so that the cell values are calculated in the right order. Faulty circular dependencies are detected and reported back to the user.

Opinions?

Uh oh!

MEP for a matplotlib geometry manager #1109

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