Rework MacOS and Linux configuration paths, to match documentation

This brings us in line with pre-platformdirs behaviours on these,
following the details of the configuration paths, as documented in pip's
own documentation.

Author: pradyunsg

src/pip/_internal/utils/appdirs.py

@@ -17,24 +17,36 @@ def user_cache_dir(appname: str) -> str:
     return _appdirs.user_cache_dir(appname, appauthor=False)
 
 
+def _macos_user_config_dir(appname: str, roaming: bool = True) -> str:
+    # Use ~/Application Support/pip, if the directory exists.
+    path = _appdirs.user_data_dir(appname, appauthor=False, roaming=roaming)
+    if os.path.isdir(path):
+        return path
+
+    # Use a Linux-like ~/.config/pip, by default.
+    linux_like_path = "~/.config/"
+    if appname:
+        linux_like_path = os.path.join(linux_like_path, appname)
+
+    return os.path.expanduser(linux_like_path)
+
+
 def user_config_dir(appname: str, roaming: bool = True) -> str:
-    path = _appdirs.user_config_dir(appname, appauthor=False, roaming=roaming)
-    if sys.platform == "darwin" and not os.path.isdir(path):
-        path = os.path.expanduser("~/.config/")
-        if appname:
-            path = os.path.join(path, appname)
-    return path
+    if sys.platform == "darwin":
+        return _macos_user_config_dir(appname, roaming)
+
+    return _appdirs.user_config_dir(appname, appauthor=False, roaming=roaming)
 
 
 # for the discussion regarding site_config_dir locations
 # see <https://github.com/pypa/pip/issues/1733>
 def site_config_dirs(appname: str) -> List[str]:
-    dirval = _appdirs.site_config_dir(appname, appauthor=False, multipath=True)
     if sys.platform == "darwin":
-        # always look in /Library/Application Support/pip as well
-        return dirval.split(os.pathsep) + ["/Library/Application Support/pip"]
-    elif sys.platform == "win32":
+        return [_appdirs.site_data_dir(appname, appauthor=False, multipath=True)]
+
+    dirval = _appdirs.site_config_dir(appname, appauthor=False, multipath=True)
+    if sys.platform == "win32":
         return [dirval]
-    else:
-        # always look in /etc directly as well
-        return dirval.split(os.pathsep) + ["/etc"]
+
+    # Unix-y system. Look in /etc as well.
+    return ["/etc"] + dirval.split(os.pathsep)

tests/unit/test_appdirs.py

@@ -99,15 +99,17 @@ def test_site_config_dirs_osx(self, monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.setenv("HOME", "/home/test")
 
         assert appdirs.site_config_dirs("pip") == [
-            "/Library/Preferences/pip",
             "/Library/Application Support/pip",
         ]
 
     @pytest.mark.skipif(sys.platform != "linux", reason="Linux-only test")
     def test_site_config_dirs_linux(self, monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.delenv("XDG_CONFIG_DIRS", raising=False)
 
-        assert appdirs.site_config_dirs("pip") == ["/etc/xdg/pip", "/etc"]
+        assert appdirs.site_config_dirs("pip") == [
+            "/etc",
+            "/etc/xdg/pip",
+        ]
 
     @pytest.mark.skipif(sys.platform != "linux", reason="Linux-only test")
     def test_site_config_dirs_linux_override(
@@ -117,10 +119,10 @@ def test_site_config_dirs_linux_override(
         monkeypatch.setenv("XDG_CONFIG_DIRS", "/spam:/etc:/etc/xdg")
 
         assert appdirs.site_config_dirs("pip") == [
+            "/etc",
             "/spam/pip",
             "/etc/pip",
             "/etc/xdg/pip",
-            "/etc",
         ]
 
     @pytest.mark.skipif(sys.platform != "linux", reason="Linux-only test")
@@ -129,7 +131,10 @@ def test_site_config_dirs_linux_empty(
     ) -> None:
         monkeypatch.setattr(os, "pathsep", ":")
         monkeypatch.setenv("XDG_CONFIG_DIRS", "")
-        assert appdirs.site_config_dirs("pip") == ["/etc/xdg/pip", "/etc"]
+        assert appdirs.site_config_dirs("pip") == [
+            "/etc",
+            "/etc/xdg/pip",
+        ]
 
 
 class TestUserConfigDir: