Add appdirs support

Closes #5

README.md

@@ -105,7 +105,7 @@ rdf_graph: rdflib.Graph = pystow.ensure_rdf('rhea', url=url)
 
 Also see `pystow.ensure_excel()`, `pystow.ensure_rdf()`, `pystow.ensure_zip_df()`, and `pystow.ensure_tar_df()`.
 
-### ⚙️️ Configuration
+## ⚙️️ Configuration
 
 By default, data is stored in the `$HOME/.data` directory. By default, the `<app>` app will create the
 `$HOME/.data/<app>` folder.
@@ -144,6 +144,18 @@ pykeen_directory = pystow.join('pykeen')
 
 Note: if you set `PYSTOW_HOME`, then `PYSTOW_NAME` is disregarded.
 
+### XGD
+
+While PyStow's main goal is to make application data less opaque and less
+hidden, some users might want to use the
+[XGD spec](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)
+for storing their app data.
+
+If you set `PYSTOW_USE_APPDIRS` to `true` or `True`, then the
+[`appdirs`](https://pypi.org/project/appdirs/) package will be used to choose
+the base directory based on the `user data dir` option. This can still be
+overridden by `PYSTOW_HOME`.
+
 ## 🚀 Installation
 
 The most recent release can be installed from

src/pystow/module.py

@@ -42,6 +42,7 @@
 
 PYSTOW_NAME_ENVVAR = "PYSTOW_NAME"
 PYSTOW_HOME_ENVVAR = "PYSTOW_HOME"
+PYSTOW_USE_APPDIRS = "PYSTOW_USE_APPDIRS"
 PYSTOW_NAME_DEFAULT = ".data"
 
 
@@ -50,9 +51,18 @@ def get_name() -> str:
     return os.getenv(PYSTOW_NAME_ENVVAR, default=PYSTOW_NAME_DEFAULT)
 
 
+def _use_appdirs() -> bool:
+    return os.getenv(PYSTOW_USE_APPDIRS) in {"true", "True"}
+
+
 def get_home(ensure_exists: bool = True) -> Path:
     """Get the PyStow home directory."""
-    default = Path.home() / get_name()
+    if _use_appdirs():
+        from appdirs import user_data_dir
+
+        default = Path(user_data_dir())
+    else:
+        default = Path.home() / get_name()
     return getenv_path(PYSTOW_HOME_ENVVAR, default, ensure_exists=ensure_exists)
 
 
@@ -71,7 +81,12 @@ def get_base(key: str, ensure_exists: bool = True) -> Path:
     """
     _assert_valid(key)
     envvar = f"{key.upper()}_HOME"
-    default = get_home(ensure_exists=False) / key
+    if _use_appdirs():
+        from appdirs import user_data_dir
+
+        default = Path(user_data_dir(appname=key))
+    else:
+        default = get_home(ensure_exists=False) / key
     return getenv_path(envvar, default, ensure_exists=ensure_exists)