[Matplotlib-devel] High priority for 3.2: figure cleanup

Benjamin Root ben.v.root at gmail.com
Tue Apr 16 20:22:48 EDT 2019

I don't think the ability to save after closing the GUI window has always
worked. In fact, I have always taught students to perform a savefig prior
to doing `plt.show()` if you want to programmatically save a figure and
display it. I'd be perfectly fine with killing off that mis-feature. To me,
closing a figure window should be equivalent to calling `fig.close()`. If
you wanted to save the figure after closing the gui window, you could
easily do it from the save file button.

On Tue, Apr 16, 2019 at 12:02 PM Antony Lee <antony.lee at institutoptique.fr>

> Hi,
> It's actually not clear to me how GUI-closing and really closing are
> related (unless we decided that GUI closing means teardown, but I don't
> think we *need* that), and what the "latent figures" concept is supposed to
> achieve.
> Doesn't this approximately come down to
> 1) when calling plt.close(fig), do fig.clf() -> ax.cla()
> 2) in ax.cla(), before doing ax.lines = [], first do for line in ax.lines:
> line.remove()? (and similarly for other artists)
> Also, note that if one just wants to dump figures to your hard drive in a
> loop, I would strongly suggest not bothering with pyplot as all and just do
> "fig = matplotlib.figure.Figure(); ax = fig.subplots(); etc."  The main
> issue with this approach is that it's a bit awkward to pop up a figure if
> one decides, after all, that they want to interactively play with it; a
> possibility would be to allow pyplot to "adopt" figures in a post-hoc
> manner via plt.show([fig1, fig2, ...]) (
> https://github.com/matplotlib/matplotlib/pull/13590#issuecomment-469497636
> ).
> Antony
> On Tue, Apr 16, 2019 at 5:41 PM Jody Klymak <jklymak at uvic.ca> wrote:
>> I think if it makes our lives easier the paradigm that a closed window
>> means the figure is closed programmatically is pretty straightforward and
>> standard across GUI interfaces.  If folks need a workflow where they access
>> the figure while it’s open they can turn off blocking.  I vote we keep it
>> simple and predictable rather than tie ourselves in knots with complicated
>> ways of keeping track of what we think the user may want to do.
>> Cheers.  Jody
>> Sent from my iPhone
>> > On Apr 16, 2019, at 12:19 AM, Eric Firing <efiring at hawaii.edu> wrote:
>> >
>> >
>> > In today's weekly meeting, with Hannah, Thomas, and myself, we
>> discussed https://github.com/matplotlib/matplotlib/issues/8519, which
>> points to the long-standing problem that matplotlib relies on garbage
>> collection to release memory after a figure is closed.  With some
>> work-flows the automatic collection doesn't happen often enough and the
>> user must explicitly trigger it.  It would be better if pyplot cleaned up
>> after itself, breaking reference cycles when a figure is closed.  I think
>> it is time to make this change, so I marked the issue "Release-critical". I
>> am hoping we can get this done as a high-priority task, *early* in the 3.2
>> cycle.
>> >
>> > The central point is that closing a figure should clear it, and
>> clearing it should recursively walk through the children, breaking
>> connections and reference cycles all the way down.  One possibility would
>> be to modify or replace the figure object such that any attempt to operate
>> on the figure after closing would fail by raising an informative exception.
>> >
>> > Question: should closing the gui window also close the figure?  No,
>> because that would prevent the user from calling savefig on it.  This would
>> interfere with a reasonable use-case, in which plt.show is used in blocking
>> mode, the window is closed, and the figure is saved via savefig.  Problem:
>> if closing the gui window doesn't close the figure, how can it ever be
>> closed?
>> >
>> > This leads to the suggestion that the pyplot state manager keep a list
>> of "latent figures" which are no longer displayed in a gui window but have
>> not been closed.  They could then be explicitly closed by
>> "plt.close('all')".  This also opens the possibility for an API allowing
>> one to re-display a figure after closing its gui window. For example, one
>> might do a calculation, show it in a figure in a blocking window with
>> "plt.show", close the window, do additional calculations, add them to the
>> same figure, and bring the figure back in a new gui window.  This might be
>> done in a loop, incrementing the contents of the figure.
>> >
>> > See also:
>> > https://github.com/matplotlib/matplotlib/issues/6793
>> > https://github.com/matplotlib/matplotlib/issues/6982
>> >
>> > Eric
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