Exclude code coverage based on operating system

[ericzolf]

Is your feature request related to a problem? Please describe.
I have code which is operating system dependent and hence can't be covered on a single operating system, which doesn't allow me to understand how well my code is covered on each single operating system.
That probably also encourages me to accept a lower than necessary code coverage as I can't anyway reach 100%, and don't know how much of the missing 14% are due to this issue.

Describe the solution you'd like
I'd like to have conditional # pragma: no cover, e.g. # pragma: no cover if os.name == "nt" or # pragma: cover if os.name == "nt" or # pragma: no cover if os.name != "nt" (the 2 last being equivalent). sys.platform or any equivalent approach resp. syntax would be fine.

Describe alternatives you've considered
I didn't find any alternative beside ignoring missing coverage "in my head".

Additional context
https://github.com/rdiff-backup/rdiff-backup/blob/master/tox.ini shows only 86% coverage among other, because src/rdiffbackup/meta/acl_win.py is only covered at 26% and src/rdiffbackup/utils/locking.py at 36%, both being very specific to Windows (and I've completely ignored the MacOS code at src/rdiffbackup/meta/stdattr.py).

I have in mind something like @unittest.skipUnless resp. @unittest.skipIf but it doesn't need to be as flexible/dynamic.

[nedbat]

I won't be adding this as a built-in feature of coverage, but there are third-party tools that do it: https://github.com/asottile/covdefaults. You can also do this yourself in a tox.ini file or similar, by defining an environment variable based on the OS, then using that variable in your .coveragerc file to specify exclude_also patterns. (I thought I knew where an example was, but I can't find it now.)

[nedbat]

The other thing to remember is that you can combine coverage results from multiple runs. Test on Windows and Unix, then combine the results to get the total.

[freakboy3742]

Requested by @nedbat on Mastodon: If you want an example of this in practice: Briefcase has an extensive set of Python version and OS conditional coverage definitions, with coverage merging in CI: https://github.com/beeware/briefcase/blob/main/pyproject.toml#L30