core/cachew: special handling for None in order to preserve cache_dir() path

+ add 'suffix' argument for more straighforward logic

my/core/cachew.py

@@ -38,6 +38,28 @@ def _appdirs_cache_dir() -> Path:
     return cd
 
 
-def cache_dir() -> Optional[Path]:
+from . import PathIsh
+def cache_dir(suffix: Optional[PathIsh] = None) -> Path:
     from . import core_config as CC
-    return CC.config.get_cache_dir()
+    cdir_ = CC.config.get_cache_dir()
+
+    sp: Optional[Path] = None
+    if suffix is not None:
+        sp = Path(suffix)
+        # guess if you do need absolute, better path it directly instead of as suffix?
+        assert not sp.is_absolute(), sp
+
+    # ok, so ideally we could just return cdir_ / sp
+    # however, this function was at first used without the suffix, e.g. cache_dir() / 'some_dir'
+    # but now cache_dir setting can also be None which means 'disable cache'
+    # changing return type to Optional means that it will break for existing users even if the cache isn't used
+    # it's kinda wrong.. so we use dummy path (_CACHE_DIR_NONE_HACK), and then strip it away in core.common.mcachew
+    # this logic is tested via test_cachew_dir_none
+
+    if cdir_ is None:
+        from .common import _CACHE_DIR_NONE_HACK
+        cdir = _CACHE_DIR_NONE_HACK
+    else:
+        cdir = cdir_
+
+    return cdir if sp is None else cdir / sp