`--no-incremental` still writes `.mypy_cache`

[asottile-sentry]

Bug Report

the help seems to indicate a cache will not be produced with --no-incremental:

$ mypy --help | grep -A2 no-inc
  --no-incremental          Disable module cache (inverse: --incremental)
  --cache-dir DIR           Store module cache info in the given folder in
                            incremental mode (defaults to '.mypy_cache')

a workaround is --cache-dir /dev/null though this isn't super portable

To Reproduce

print(1)

rm -rf .mypy_cache
mypy t.py --no-incremental
ls -ald .mypy_cache

Expected Behavior

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -ald .mypy_cache
ls: cannot access '.mypy_cache': No such file or directory

Actual Behavior

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -ald .mypy_cache
drwxr-xr-x 5 asottile wheel 160 Jul 22 07:36 .mypy_cache

Your Environment

• Mypy version used: 1.17.0
• Mypy command-line flags: --no-incremental
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used:

[emmatyping]

This is actually documented as the current behavior: https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-cache-dir

However I do agree having a portable flag to disable writing the cache is probably a good idea.

[asottile-sentry]

ah neat -- seems an opportunity to improve --help as well to match the docs (perhaps s/in/for/ from the excerpt above would have been enough for me to dig further) and maybe "Disable reading module cache" for --no-incremental

[wyattscarpenter]

It's not relevant to your main request, but the documentation issue is yet another problem #12347 would fix. (But I wouldn't hold my breath for that)