I'm wondering if this maybe should always (re)create the CACHEDIR.TAG file in .mypy_cache to avoid situations where Mypy is interrupted after creating the directory and before the file is created, leaving the directory CACHEDIR.TAG-less forever. I estimate the chance of this to be slim, but given enough people using it there'll be a handful who'll encounter that condition I image and the code would not be much more complicated, the relevant lines would just read

if os.path.isdir(options.cache_dir):
    exclude_from_backups(options.cache_dir)

Following up on the concern mentioned above, we could attempt to "create" the directory + gitignore + CACHEDIR.TAG atomically by creating a temporary directory, creating files in it and then renaming the temporary directory to the desired path (like I did in a patch to Cargo) but this would have to happen somewhere deeper down the stack, in the place where the actual cache directory is created now (I'm not sure where is it).

FWIW I had a quick look around and the only other open source backup solution I found (I only looked at open source ones) that supports this out of the box is attic, others (like rsync and bacula) support this in a exclude-directory-if-a-file-named-like-this-exists-in-it way – so while my initial commit message may be technically correct I wanted the above to be known, I don't want to oversell this solution on false premises.

Cheers.