Switch matplotlib backend based on OS

[fbrausse]

For macos use "macosx", for Linux use "tkagg".

[fbrausse]

What's matplotlib's default if one doesn't explicitly set the backend?

[konstantin-korovin]

Good point! 1. on MacOS: python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()" DEBUG:matplotlib.pyplot:Loaded backend macosx version unknown. And it does open a window. 2. On MacOS docker python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()" <string>:1: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure. So at least on Mac commenting the backend line does the right thing. On Linux ?

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On Fri, 20 Mar 2026 at 11:10, Franz Brauße ***@***.***> wrote: *fbrausse* left a comment (SMLP-Systems/smlp#66) <#66 (comment)> What's matplotlib's default if one doesn't explicitly set the backend? — Reply to this email directly, view it on GitHub <#66?email_source=notifications&email_token=AIVYHTNVQQI2WXS3L3EBMBD4RURJ5A5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMBZG4ZDSNRTGIYKM4TFMFZW63VKON2WE43DOJUWEZLEUVSXMZLOOSWGM33PORSXEX3DNRUWG2Y#issuecomment-4097296320>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AIVYHTMMXTREQ7TUQ7QMG634RURJ5AVCNFSM6AAAAACWY6YX4WVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAOJXGI4TMMZSGA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[fbrausse]

For me on Linux without docker opens a plot window and prints:

DEBUG:matplotlib.pyplot:Loaded backend gtk3agg version 3.24.51.

(and plenty of non-related debug messages)

[fbrausse]

Without envvar DISPLAY being set:

$ env -u DISPLAY python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()"
DEBUG:matplotlib.pyplot:Loaded backend agg version v2.2.

and no window nor any errors appear, just quits with exit code 0.

[konstantin-korovin]

OK, looks reasonable. Not sure why DISPLAY needs to be set by default though. agg is non-interactive so just generates png files. I guess @zurab Khasidashvili ***@***.***> added the backend line, @zurab what happens on your system if you run 1. python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()" 2. env -u DISPLAY python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()"

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On Fri, 20 Mar 2026 at 11:37, Franz Brauße ***@***.***> wrote: *fbrausse* left a comment (SMLP-Systems/smlp#66) <#66 (comment)> Without envvar DISPLAY being set: $ env -u DISPLAY python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()" DEBUG:matplotlib.pyplot:Loaded backend agg version v2.2. and no window nor any errors appear, just quits with exit code 0. — Reply to this email directly, view it on GitHub <#66?email_source=notifications&email_token=AIVYHTIJC6ZE7THW3R4DJLD4RUUPZA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMBZG42DCNZVGY32M4TFMFZW63VHMNXW23LFNZ2KKZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#issuecomment-4097417567>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AIVYHTLGJOD75DWEHBFPENL4RUUPZAVCNFSM6AAAAACWY6YX4WVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAOJXGQYTONJWG4> . You are receiving this because you commented.Message ID: ***@***.***>

[fbrausse]

Without DISPLAY, there is no connection to the X server and therefore no GUI.

[konstantin-korovin]

But on Linux don't you have X server running? Or you run from docker ?

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On Fri, 20 Mar 2026 at 11:48, Franz Brauße ***@***.***> wrote: *fbrausse* left a comment (SMLP-Systems/smlp#66) <#66 (comment)> Without DISPLAY, there is no connection to the X server and therefore no GUI. — Reply to this email directly, view it on GitHub <#66?email_source=notifications&email_token=AIVYHTIJ6QMJFDWTK6JVGX34RUVZJA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMBZG42DONJTHE22M4TFMFZW63VHMNXW23LFNZ2KKZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#issuecomment-4097475395>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AIVYHTKW5WBI5N2AUI5B7I34RUVZJAVCNFSM6AAAAACWY6YX4WVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAOJXGQ3TKMZZGU> . You are receiving this because you commented.Message ID: ***@***.***>

[fbrausse]

Unsetting DISPLAY is just make-believe for matplotlib here: pretend there is no graphical environment, please show me what you will do then. Apparently just use the agg backend. What docker does I cannot say, sorry.

[konstantin-korovin]

OK, so commenting the backend line makes sense, as I can see.

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On Fri, 20 Mar 2026 at 11:53, Franz Brauße ***@***.***> wrote: *fbrausse* left a comment (SMLP-Systems/smlp#66) <#66 (comment)> Unsetting DISPLAY is just make-believe for matplotlib here: pretend there is no graphical environment, please show me what you will do then. Apparently just use the agg backend. What docker does I cannot say, sorry. — Reply to this email directly, view it on GitHub <#66?email_source=notifications&email_token=AIVYHTLP5BOPDGXWFMX2OXD4RUWKDA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMBZG42DSNRZG422M4TFMFZW63VHMNXW23LFNZ2KKZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#issuecomment-4097496975>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AIVYHTPJNN3F6DC72SJ6EXD4RUWKDAVCNFSM6AAAAACWY6YX4WVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAOJXGQ4TMOJXGU> . You are receiving this because you commented.Message ID: ***@***.***>

[fbrausse]

So at least on Mac commenting the backend line does the right thing.
On Linux ?

For me as well. @mdmitry1, @zurabksmlp Could you quickly check what's happening when you run Konstantin's simple test:

python3 -c "import matplotlib; from pylab import *; set_loglevel('debug'); plot(); show()"

(Edit: also inside docker)

Maybe we can just remove explicitly setting matplotlib's backend.

[mdmitry1]

Added test for default matplotlib backend.

git branch --show-current
smlptech
$(git rev-parse --show-toplevel)/tests/install/test_matplotlib_backend

Got predicted results: TkAgg has been chosen for both use cases: VNC server and virtual dispay

1 python311-dev matplotlib backend for VNC server: VNC server has started successfully with screen resolution 1280x1024 TkAgg
2 mdmitry1/python311-dev matplotlib backend for VNC server: VNC server has started successfully with screen resolution 1280x1024 TkAgg
3 smlp-test-build-almalinux_9-python311 matplotlib backend for virtual display: TkAgg
4 mdmitry1/smlp-test-build-almalinux_9-python311 matplotlib backend for virtual display: TkAgg
5 smlp-test-build-opensuse_15.5-python311 matplotlib backend for virtual display: TkAgg
6 mdmitry1/smlp-test-build-opensuse_15.5-python311 matplotlib backend for virtual display: TkAgg
7 smlp-test-build-ubuntu_22.04-python311 matplotlib backend for virtual display: TkAgg
8 mdmitry1/smlp-test-build-ubuntu_22.04-python311 matplotlib backend for virtual display: TkAgg
9 mdmitry1/python311-dev matplotlib backend for virtual display: TkAgg
10 python311-dev matplotlib backend for virtual display: TkAgg