MEP27: decouple pyplot from backends

.. contents::

:local:

Status

**Discussion**

Branches and Pull requests

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

Abstract

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 class ``FigureManager`` and a backend specific class

``WindowBase`` and

2. ``ShowBase`` and its derived classes into ``Gcf.show_all`` and ``MainLoopBase``.

Detailed description

This MEP aims to consolidate the backends API into one single uniform

API, removing generic code out of the backend (which includes

``_pylab_helpers`` and ``Gcf``), and push code to a more appropriate

level in matplotlib. With this we automatically remove

inconsistencies that appear in the backends, such as

``FigureManagerBase.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 from

``FigureManagerBase`` and ``ShowBase``.

1. ``FigureManagerBase`` has **three** 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: ```python

self.set_window_title("Figure %d" % num) ``` This combines

backend specific code ``self.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 specific

``mainloop`` to block the main programme and thus keep the

figures from dying.

Implementation

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

1. To remove the windowing aspect out of ``FigureManagerBase`` letting

it simply wrap this new class along with the other backend classes.

Create a new ``WindowBase`` class that can handle this

functionality, with pass-through methods (:arrow_right:) to

``WindowBase``. Classes that subclass ``WindowBase`` should also

subclass the GUI specific window class to ensure backward

compatibility (``manager.window == manager.window``).

2. Refactor the mainloop of ``ShowBase`` into ``MainLoopBase``, which

encapsulates the end of the loop as well. We give an instance of

``MainLoop`` to ``FigureManager`` 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 that ``FigureManagerBase`` has no backend specifics in it, to

rename it to ``FigureManager``, and move to a new file

``backend_managers.py`` noting that:

1. This allows us to break up the conversion of backends into

separate PRs as we can keep the existing ``FigureManagerBase``

class and its dependencies intact.

2. and this also anticipates MEP22 where the new

``NavigationBase`` has morphed into a backend independent

``ToolManager``.

|FigureManagerBase(canvas, num) |FigureManager(figure, num) |``WindowBase(title)``|Notes |

| | | | |

|show |:arrow_right: |show | |

|destroy |calls destroy on all |destroy | |

| |components | | |

|full_screen_toggle |handles logic |set_fullscreen | |

|resize |:arrow_right: |resize | |

|key_press |key_press |:no_entry: | |

|show_popup |show_poup |:no_entry: |Not used anywhere in mpl, and |

| | | |does nothing. |

| | | | |

| | | | |

| | | | |

|get_window_title |:arrow_right: |get_window_title | |

|set_window_title |:arrow_right: |set_window_title | |

|:no_entry: |_get_toolbar | |A common method to all |

| | | |subclasses of FigureManagerBase |

| | | | |

| | | | |

|:no_entry: |:no_entry: |set_default_size | |

|:no_entry: |:no_entry: |add_element_to_window| |

|ShowBase |MainLoopBase|Notes |

|mainloop |begin | |

|:no_entry:|end |Gets called |

| | |automagically|

| | |when no more |

| | |instances of |

| | |the subclass |

| | |exist |

|__call__ |:no_entry: |Method moved |

| | |to |

| | |Gcf.show_all |

Future compatibility

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 from ``FigureManagerBase``. With

this code, this only needs to get made in the single ``FigureManager``

class. This also makes the later deprecation of

``NavigationToolbar2`` very straightforward, only needing to touch the

single ``FigureManager`` class

MEP 23 makes for another use case where this refactored code will come

in very handy.

Backward compatibility

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 used

``FigureManager.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 as

``NavigationToolbar2``, also note that the change in call signature to

the ``FigureCanvasWx`` constructor, while backward compatible, I think

the old (imho ugly style) signature should get deprecated and removed

in the same manner as everything else.

|backend |manager.resize(w,h) |Extra |

|gtk3 |window | |

|Tk |canvas | |

|Qt |window | |

|Wx |canvas |FigureManagerWx had |

| | |``frame`` as an alias to |

| | |window, so this also |

| | |breaks BC. |

Alternatives

If there were any alternative solutions to solving the same problem,

they should be discussed here, along with a justification for the

chosen approach.

Questions

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 the ``MainLoopBase``

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 of ``FigureManager`` 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.

MEP27