[Matplotlib-users] Simultaneous plots with Matplotlib "QObject/thread" error

[Eric Firing]

John,

Your new approach looks good to me, but could be tweaked.  You don't 
need the 'fig.clf()' at the end because the 'plt.close(fig)' will take 
care of cleaning everything up.  At the start, using 'fig.gca()' seems a 
little odd.  A common and compact idiom now would be 'fig, ax = 
plt.subplots()'.  Once you have the figure reference, you don't need to 
recreate it with the call to 'ax.get_figure()`, so you can delete that. 
I don't think you need both tight_layout and bbox_inches='tight'. My 
impression is that the latter is more robust, but has the possible 
disadvantage of modifying the figure dimensions, which might be 
undesirable in a web app.

Eric

On 2016/09/09 11:24 AM, John Robson wrote:
> Eric, you are right, I followed this and worked:
>
> http://matplotlib.org/faq/howto_faq.html#howto-webapp
>
> I also fixed other problems, I was using "plt.savefig", "plt.xxx" I
> believe that this "plt" was harming other simultaneous plots, so I
> replaced every "plt." by ".fig" and now I think is OK.
>
> I just wonder how can I isolate each plot from other simultaneous plots,
> I'm doing this:
>
> fig = plt.figure()
> ax = fig.gca()
>> plot stuff, scatterplot, etc..., set title, lables, etc...
> fig = ax.get_figure()
> fig.set_tight_layout(True)
> fig.savefig(filename, format='png', dpi=100, facecolor='w',
> edgecolor='k', bbox_inches='tight')
> fig.clf()
> plt.close(fig)
>
> So the first "fig" instance is destroyed at the end and there is no
> "plt.xxx"
>
> This is a good approach to isolate all plots?
>
> Thank you,
> John
>
>
>
> On 09/09/16 03:16, Eric Firing wrote:
>> On 2016/09/08 8:40 PM, John Robson wrote:
>>> Hi all,
>>>
>>> I have some Flask pages that plot charts using Matplotlib, they work
>>> very well, but when running them simultaneously they break the site,
>>> with errors like:
>>>
>>> "QObject: Cannot create children for a parent that is in a different
>>> thread."
>>>
>>> "QObject::~QObject: Timers cannot be stopped from another thread"
>>>
>>> I wonder how to safely use Matplolib with Flask (with several
>>> simultaneous users plotting charts all the time)?
>>
>> I suspect the problem is that matplotlib is running with the qt4agg
>> backend.
>>
>> When plotting for a web app like that, you need to make sure the backend
>> is set to the agg backend.  You can do that in a matplotlibrc file or
>> directly in the plotting script, immediately after importing matplotlib
>> and before importing pyplot (if you use pyplot at all).
>>
>> Eric
>>
>>>
>>> Thank you,
>>> John
>
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