Details
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Bug
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Resolution: Out of scope
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Not Evaluated
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None
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6.8.2
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None
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o LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 12 (bookworm)
Release: 12
Codename: bookworm
(6.86x)
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Linux/X11
Description
I'm using matplotlib with python, which relies on QT for displaying plots. I'm logged into a remote workstation, using ssh -X. I'm running a python file which imports matplotlib and displays some plots on the screen.
The plots work well for some time, but after a while and seemingly at random, the plots stop working. Without changing anything, I suddenly start seeing errors like:
ImportError: Cannot load backend 'QtAgg' which requires the 'qt' interactive framework, as 'headless' is currently running
This magically resolves itself by exiting the `ssh X` session and logging in again.
I tried adding some logging to try to understand why matplotlib thinks the current backend is "headless" instead of "qt", this is what I found:
QtWidgets = ( sys.modules.get("PyQt6.QtWidgets") or sys.modules.get("PySide6.QtWidgets") or sys.modules.get("PyQt5.QtWidgets") or sys.modules.get("PySide2.QtWidgets") ) print("DDEBUG cbook.py vgk3: value of QtWidgets: {}".format(QtWidgets)) instance = QtWidgets.QApplication.instance() print("DDEBUG cbook.py 8y0p: value of instance: {}".format(instance)) if QtWidgets and QtWidgets.QApplication.instance(): return "qt"
DDEBUG cbook.py vgk3: value of QtWidgets: <module 'PyQt6.QtWidgets' from '/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/PyQt6/QtWidgets.abi3.so'> DDEBUG cbook.py 8y0p: value of instance: None DDEBUG pyplot.py n57x: value of current_framework: headless DDEBUG pyplot.py fr4q: value of required_framework: qt Traceback (most recent call last): File "/home/USER/git/6.86x/project1/sentiment_analysis/./main.py", line 55, in <module> plot_toy_results('Perceptron', thetas_perceptron) ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/USER/git/6.86x/project1/sentiment_analysis/./main.py", line 53, in plot_toy_results utils.plot_toy_data(algo_name, toy_features, toy_labels, thetas) ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/USER/git/6.86x/project1/sentiment_analysis/utils.py", line 102, in plot_toy_data plt.subplots() ~~~~~~~~~~~~^^ File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 1777, in subplots fig = figure(**fig_kw) File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 1044, in figure manager = new_figure_manager( num, figsize=figsize, dpi=dpi, facecolor=facecolor, edgecolor=edgecolor, frameon=frameon, FigureClass=FigureClass, **kwargs) File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 553, in new_figure_manager _warn_if_gui_out_of_main_thread() ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^ File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 530, in _warn_if_gui_out_of_main_thread canvas_class = cast(type[FigureCanvasBase], _get_backend_mod().FigureCanvas) ~~~~~~~~~~~~~~~~^^ File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 369, in _get_backend_mod switch_backend(rcParams._get("backend")) ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/USER/anaconda3/envs/6.86x/lib/python3.13/site-packages/matplotlib/pyplot.py", line 435, in switch_backend raise ImportError( ...<2 lines>... newbackend, required_framework, current_framework)) ImportError: Cannot load backend 'QtAgg' which requires the 'qt' interactive framework, as 'headless' is currently running
matplotlib's cbook.py checks that a QT module can be imported, and that QtWidgets.QApplication.instance() is truthy. When this bug is being reproduced, the latter is false, i.e. QtWidgets.QApplication.instance() is None.
I'm not sure if I can dig down any further since I'm not familiar with QT internals and the _file_ for QT modules in python refer to .so libraries.
What further debugging steps do you recommend for helping us understand why QtWidgets.QApplication.instance() mysteriously becomes None after a while within an ssh -X session?