An alternative way to do this which would shift the error handling upstream, would be call path.resolve(strict=True), which will raise an error if the path doesn't exist (https://docs.python.org/3/library/pathlib.html?highlight=pathlib#pathlib.Path.resolve)

[Side point: I did not know about the Path.resolve() trick to move error generation to the stdlib itself; it is nice because you can actually get the FileNotFoundError, PermissionError, or whatnot. (Perhaps the call should really be Path.stat(), though, as can be seen by looking at the implementation of Path.exists()). From a quick skim through the library, I think the only other place where we could switch to using it is at the start of matplotlib.testing.compare.convert().]

It was indeed real... Wasn't sure if it was local so went ahead.

The class structure could really need an update. Now, one cannot call super().setup() as that calls _run as part of setup, which is not possible for a FileWriter class.

OK, this should now be good to go. OS-dependent tests added.