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pllim opened this issue Apr 1, 2021 · 8 comments · Fixed by #19855
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ValueError: Key backend: '' is not a valid value for backend #19848

pllim opened this issue Apr 1, 2021 · 8 comments · Fixed by #19855
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions.
Milestone
v3.5.0

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@pllim
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pllim commented Apr 1, 2021 •
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Bug report

I am actually not sure if this is a bug. It started happening today for matplotlib installed from your master branch for our CI. We run some image tests using your dev version on CircleCI, which uses our tox.ini and some settings in astropy/conftest.py.

It starts to complain about no backend even though we set it at astropy/conftest.py#L84 and that works for older versions of matplotlib.

INTERNALERROR>   File ".../matplotlib/__init__.py", line 843, in <module>
INTERNALERROR>     fail_on_error=True)
INTERNALERROR>   File ".../matplotlib/__init__.py", line 773, in _rc_params_in_file
INTERNALERROR>     config[key] = val  # try to convert to proper type or raise
INTERNALERROR>   File ".../matplotlib/__init__.py", line 607, in __setitem__
INTERNALERROR>     raise ValueError(f"Key {key}: {ve}") from None
INTERNALERROR> ValueError: Key backend: '' is not a valid value for backend; supported values are
['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo',
'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']

Code for reproduction

We can reproduce it on both GitHub Actions and CircleCI using tox + pytest but only with the dev version of matplotlib.

Actual outcome

See traceback above.

Expected outcome

It does not throw ValueError.

Matplotlib version

Also, the Actions log.

xref astropy/astropy#11458
@pllim pllim mentioned this issue Apr 1, 2021
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@QuLogic
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QuLogic commented Apr 1, 2021

Probably related to #19603

@pllim
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pllim commented Apr 2, 2021

@QuLogic , what does that mean for downstream setup? Do we have to completely change the way we set backend for CI? Can you please advise? Thanks!

@ianhi
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ianhi commented Apr 2, 2021

I ran into this yesterday as well and opened: matplotlib/pytest-mpl#117

For me this was failing just from using pytest-mpl before getting to any tests:

INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/pytest_mpl/plugin.py", line 124, in pytest_report_header
INTERNALERROR>     import matplotlib
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/matplotlib/__init__.py", line 840, in <module>
INTERNALERROR>     rcParamsDefault = _rc_params_in_file(
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/matplotlib/__init__.py", line 774, in _rc_params_in_file
INTERNALERROR>     config[key] = val  # try to convert to proper type or raise
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.9.2/x64/lib/python3.9/site-packages/matplotlib/__init__.py", line 608, in __setitem__
INTERNALERROR>     raise ValueError(f"Key {key}: {ve}") from None
INTERNALERROR> ValueError: Key backend: '' is not a valid value for backend; supported values are ['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo', 'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo', 'pdf', 'pgf', 'ps', 'svg', 'template']

@pllim
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pllim commented Apr 2, 2021

It is unclear to me where the fix should be. Does pytest-mpl need to be fixed to be compatible now, or is this unintentional side effect of matplotlib change?

@jklymak
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jklymak commented Apr 2, 2021

I'm pretty sure it's unintentional and as @QuLogic suggested probably caused by the merged change in the template handling. @anntzer is the expert on the fallback logic and maybe can tease it apart.

@tacaswell tacaswell added this to the v3.5.0 milestone Apr 2, 2021
@tacaswell tacaswell added the Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. label Apr 2, 2021
@tacaswell
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This is definitely a fix on our side, I will look at this this afternoon.

@ianhi ianhi mentioned this issue Apr 2, 2021
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@pllim
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pllim commented Apr 2, 2021

Thank you all for the quick responses!

tacaswell added a commit to tacaswell/matplotlib that referenced this issue Apr 2, 2021
@tacaswell
FIX: ignore empty backend value in matplotlibrc
7264112 
See comment in code.  Accounts for the fact that we can to directly specify the
"fallback" backend value.  This interprets the lack of a backend entry and an
empty backend entry the same.

closes matplotlib#19848
@tacaswell
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See #19854

@anntzer anntzer mentioned this issue Apr 2, 2021
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@johnflavin johnflavin mentioned this issue Apr 4, 2021
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@jcohenadad jcohenadad mentioned this issue Jun 8, 2024
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions.
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Correct handle default backend. anntzer/matplotlib
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@tacaswell @QuLogic @jklymak @pllim @ianhi