.. contents:: Table of Contents

.. author:: OceanWolf

.. date:: March 2, 2015

Progress

Main PR (including GTK3):

• https://github.com/matplotlib/matplotlib/pull/4143

Backend specific branch diffs:

• https://github.com/OceanWolf/matplotlib/compare/backend-refactor...OceanWolf:backend-refactor-tkagg
• https://github.com/OceanWolf/matplotlib/compare/backend-refactor...OceanWolf:backend-refactor-qt
• https://github.com/OceanWolf/matplotlib/compare/backend-refactor...backend-refactor-wx

This MEP refactors the backends to give a more structured and consistent API, removing generic code and consolidate existing code. To do this we propose splitting:

1. FigureManagerBase and its derived classes into the core functionality classFigureManager and a backend specific classWindowBase and
2. ShowBase and its derived classes intoGcf.show_all andMainLoopBase.

This MEP aims to consolidate the backends API into one single uniform API, removing generic code out of the backend (which includes_pylab_helpers andGcf), and push code to a more appropriate level in matplotlib. With this we automatically remove inconsistencies that appear in the backends, such asFigureManagerBase.resize(w, h) which sometimes sets the canvas, and other times set the entire window to the dimensions given, depending on the backend.

Two main places for generic code appear in the classes derived fromFigureManagerBase andShowBase.

1. FigureManagerBase hasthree jobs at the moment:
   1. The documentation describes it as a``Helper class for pyplot mode, wraps everything up into a neat bundle''
   2. But it doesn't just wrap the canvas and toolbar, it also does all of the windowing tasks itself. The conflation of these two tasks gets seen the best in the following line:

self.set_window_title("Figure %d" % num)```This combines backend specific codeself.set_window_title(title) with matplotlib generic code ``title = "Figure %d" % num``.

3. Currently the backend specific subclass of ``FigureManager`` decides when to end the mainloop.  This also seems very wrong as the figure should have no control over the other figures.

2. ShowBase has two jobs:
   1. It has the job of going through all figure managers registered in_pylab_helpers.Gcf and telling them to show themselves.
   2. And secondly it has the job of performing the backend specificmainloop to block the main programme and thus keep the figures from dying.

The description of this MEP gives us most of the solution:

1. To remove the windowing aspect out ofFigureManagerBase letting it simply wrap this new class along with the other backend classes. Create a newWindowBase class that can handle this functionality, with pass-through methods (:arrow_right:) toWindowBase. Classes that subclassWindowBase should also subclass the GUI specific window class to ensure backward compatibility (manager.window == manager.window).
2. Refactor the mainloop ofShowBase intoMainLoopBase, which encapsulates the end of the loop as well. We give an instance ofMainLoop toFigureManager as a key unlock the exit method (requiring all keys returned before the loop can die). Note this opens the possibility for multiple backends to run concurrently.
3. Now thatFigureManagerBase has no backend specifics in it, to rename it toFigureManager, and move to a new filebackend_managers.py noting that:
   1. This allows us to break up the conversion of backends into separate PRs as we can keep the existingFigureManagerBase class and its dependencies intact.
   2. and this also anticipates MEP22 where the newNavigationBase has morphed into a backend independentToolManager.

As eluded to above when discussing MEP 22, this refactor makes it easy to add in new generic features. At the moment, MEP 22 has to make ugly hacks to each class extending fromFigureManagerBase. With this code, this only needs to get made in the singleFigureManager class. This also makes the later deprecation ofNavigationToolbar2 very straightforward, only needing to touch the singleFigureManager class

MEP 23 makes for another use case where this refactored code will come in very handy.

As we leave all backend code intact, only adding missing methods to existing classes, this should work seamlessly for all use cases. The only difference will lie for backends that usedFigureManager.resize to resize the canvas and not the window, due to the standardisation of the API.

I would envision that the classes made obsolete by this refactor get deprecated and removed on the same timetable asNavigationToolbar2, also note that the change in call signature to theFigureCanvasWx constructor, while backward compatible, I think the old (imho ugly style) signature should get deprecated and removed in the same manner as everything else.

If there were any alternative solutions to solving the same problem, they should be discussed here, along with a justification for the chosen approach.

Mdehoon: Can you elaborate on how to run multiple backends concurrently?

OceanWolf: @mdehoon, as I say, not for this MEP, but I see this MEP opens it up as a future possibility. Basically theMainLoopBase class acts a per backend Gcf, in this MEP it tracks the number of figures open per backend, and manages the mainloops for those backends. It closes the backend specific mainloop when it detects that no figures remain open for that backend. Because of this I imagine that with only a small amount of tweaking that we can do full-multi-backend matplotlib. No idea yet why one would want to, but I leave the possibility there in MainLoopBase. With all the backend-code specifics refactored out ofFigureManager also aids in this, one manager to rule them (the backends) all.

Mdehoon: @OceanWolf, OK, thanks for the explanation. Having a uniform API for the backends is very important for the maintainability of matplotlib. I think this MEP is a step in the right direction.