bug#70687: python-matplotlib not respecting env var MPLBACKEND=TkAgg

bug#70687: python-matplotlib not respecting env var MPLBACKEND=TkAgg

[Jake]

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Hello

We are supposed to be able to control the backend used by Python's
Matplotlib via the MPLBACKEND environment variable [1]. This doesn't appear
to work for the TkAgg backend:

#+begin_src sh

  guix shell python python-matplotlib -- bash -c 'MPLBACKEND=TkAgg python3
-c "import matplotlib; print(matplotlib.get_backend()); import
matplotlib.pyplot as plt; plt.plot([1],[2]); plt.show()"'

#+end_src

#+RESULTS:
: TkAgg
: <string>:1: UserWarning: FigureCanvasAgg is non-interactive, and thus
cannot be shown

However, the TkAgg backend does work if it is hardcoded in the Python
script using `matplotlib.use()`:

#+begin_src sh

  guix shell python python-matplotlib -- python3 -c "import matplotlib;
matplotlib.use('TkAgg'); print(matplotlib.get_backend()); import
matplotlib.pyplot as plt; plt.plot([1],[2]); plt.show()"

#+end_src

#+RESULTS:
: TkAgg

Thanks
Jake

[1]: https://matplotlib.org/stable/users/explain/figure/backends.html

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