matplotlib use non-interactive agg backend

[DanielYang59]

Fix a previously added parameter as the original comment is a bit misleading, agg is the recommended non-interactive backend:

The last, Agg, is a non-interactive backend that can only write to files. It is used on Linux, if Matplotlib cannot connect to either an X display or a Wayland display.

[DanielYang59]

No this is not right, as running tests locally would behave differently than through GH action, would fix this later

[DanielYang59]

I realized the original method might be the best I could do so far. If we were to export the env var from the test workflow config file python-package.yml, it would only be effective running on actions (not locally).

However I believe having that MPLBACKEND set is still beneficial:

• The Windows backend tk for Github runner is pretty flaky at least for pymatgen, matplotlib test use non-interactive agg backend  materialsproject/pymatgen#4168
• I believe it's recommended to use Agg for non-interactive purpose anyway

pytest doesn't seem to natively support setting environment variable from pyproject.toml and there is a plugin pytest-env just for this purpose (though with the price of added an additional tests dependency):

[tool.pytest_env]
HOME = "~/tmp"
RUN_ENV = 1

[DanielYang59]

I guess we don't need to install typing now, it's built in from 3.5:

Added in version 3.5.

[naik-aakash]

Hi @DanielYang59 , before we merge, can you please provide a bit more context on why this change is needed and what are the issues when we don't have this?

I am not that familiar with lot of what matplotlib has to offer. So just want to understand bit better 😄

And thanks in anycase for this PR

[DanielYang59]

can you please provide a bit more context on why this change is needed and what are the issues when we don't have this?

Absolutely, I owe you an explanation here:

• agg is the recommended non-interactive backend in the first place, so in a non-interactive scenario (unit test/batch image generation script), it should be used
• The reason for specifying a backend for lobsterpy is lobsterpy check the image size in test, but the image size has different rounding behaviour on MacOS (macosx backend), leading to test failure
• More practical reason is the installation of tk (default windows backend) seem to fail intermittently for GitHub Windows runners (at least for pymatgen), and would result in intermittent test failure

[naik-aakash]

Thanks @DanielYang59

[DanielYang59]

No problem thanks for reviewing