Python tests: use black and isort for formatting #24759

There doesn't seem to be an agreed upon code formatter for the files in tests/.

Some widely used formatters are black, which does general code formatting, and isort, which sorts the imports. Example usage:

black .
isort . --profile black

This could reduce time spent reviewing and discussing code style.

Generally, if the style is not mentioned nor enforced by the developer notes, we leave it up to the original author to pick whatever fits them best personally and then leave it that way until the line is touched for other reasons.

Enforcing the code style is, as you mentioned not agreed upon. One issue would be that it forces every contributor to use a dev environment that formats the code. Another one is that our current tests reformatted with black don't look great, as black enforces short lines. (Fixing this would require large-scale refactoring of most of the tests)

I don't think there is any issue to be fixed here. As mentioned previously, we leave the style up to the original author. Thus, no time is wasted on style during review. Of course, anyone is free to use black, if they like for their own code they submit.

Maybe it could make sense to mention black in our docs?

diff --git a/doc/productivity.md b/doc/productivity.md
index a01c6f545d..c05310d5b5 100644
--- a/doc/productivity.md
+++ b/doc/productivity.md
@@ -123,7 +123,13 @@ Writing code
 
 See [contrib/devtools/README.md](/contrib/devtools/README.md#clang-format-diff.py).
 
-### Format Python diffs with `yapf-diff.py`
+### Format Python
+
+#### Format Python files
+
+For newly created test files, contributors are free to (but not required to) format the whole file with the Python formatter `black`.
+
+#### Format Python diffs with `yapf-diff.py`
 
 Usage is exactly the same as [`clang-format-diff.py`](#format-cc-diffs-with-clang-format-diffpy). You can get it [here](https://github.com/MarcoFalke/yapf-diff).
 

isort might be good to mention too. Since it just sorts imports, I think it's pretty harmless. If applied to existing test files, the diff probably wouldn't be very big. Also it might reduce the number of times reviewers have to bring it up, like here. I don't know if that happens very often though.

our current tests reformatted with black don't look great, as black enforces short lines. (Fixing this would require large-scale refactoring of most of the tests)

Agree with @MarcoFalke about black. I use it sometimes as a check but it generally makes too many changes to use as-is.