llm.user_dir() creates directory if needed, closes #275

Would have fixed this bug too:
- https://github.com/simonw/llm-sentence-transformers/issues/9

docs/plugins/plugin-utilities.md

@@ -9,7 +9,7 @@ LLM stores various pieces of logging and configuration data in a directory on th
 
 On macOS this directory is `~/Library/Application Support/io.datasette.llm`, but this will differ on other operating systems.
 
-The `llm.user_dir()` function returns the path to this directory as a `pathlib.Path` object.
+The `llm.user_dir()` function returns the path to this directory as a `pathlib.Path` object, after creating that directory if it does not yet exist.
 
 Plugins can use this to store their own data in a subdirectory of this directory.
 

llm/__init__.py

@@ -188,8 +188,11 @@ def load_keys():
 def user_dir():
     llm_user_path = os.environ.get("LLM_USER_PATH")
     if llm_user_path:
-        return pathlib.Path(llm_user_path)
-    return pathlib.Path(click.get_app_dir("io.datasette.llm"))
+        path = pathlib.Path(llm_user_path)
+    else:
+        path = pathlib.Path(click.get_app_dir("io.datasette.llm"))
+    path.mkdir(exist_ok=True, parents=True)
+    return path
 
 
 def set_alias(alias, model_id_or_alias):

tests/test_llm.py

@@ -1,5 +1,6 @@
 from click.testing import CliRunner
 import datetime
+import llm
 from llm.cli import cli
 from llm.migrations import migrate
 import json
@@ -354,3 +355,12 @@ def test_llm_models_options(user_path):
     result = runner.invoke(cli, ["models", "--options"], catch_exceptions=False)
     assert result.exit_code == 0
     assert EXPECTED_OPTIONS.strip() in result.output
+
+
+def test_llm_user_dir(tmpdir, monkeypatch):
+    user_dir = str(tmpdir / "u")
+    monkeypatch.setenv("LLM_USER_PATH", user_dir)
+    assert not os.path.exists(user_dir)
+    user_dir2 = llm.user_dir()
+    assert user_dir == str(user_dir2)
+    assert os.path.exists(user_dir)