Allow reading Heroku API key from file

Makes using it in an installed environment much simpler

heroku_audit/client.py

@@ -1,4 +1,3 @@
-import os
 import sys
 from typing import Any, cast
 
@@ -7,6 +6,8 @@
 from rich import print
 from rich.text import Text
 
+from heroku_audit.config import config_env_default
+
 __all__ = ["heroku"]
 
 
@@ -19,16 +20,18 @@ class LazyHerokuWrapper:
 
     def _get_heroku(self) -> Heroku:
         if self._heroku is None:
-            try:
-                self._heroku = heroku3.from_key(os.environ["HEROKU_API_KEY"])
-            except KeyError:
+            api_key = config_env_default("heroku-api-key.txt", "HEROKU_API_KEY")
+            if api_key is None:
                 print(
                     Text(
                         "Please set $HEROKU_API_KEY to a valid Heroku API key.",
                         style="red",
                     )
                 )
                 sys.exit(1)
+
+            self._heroku = heroku3.from_key(api_key.strip())
+
         return self._heroku
 
     def __getattr__(self, attr_name: str) -> Any:

heroku_audit/config.py

@@ -0,0 +1,29 @@
+import os
+from pathlib import Path
+from typing import Optional
+
+from typer import get_app_dir
+
+APP_DIR = Path(get_app_dir("heroku-audit"))
+
+
+def config_env_default(
+    file_name: str, env_var: str, default: Optional[str] = None
+) -> Optional[str]:
+    """
+    Attempt to load a configuration value.
+
+    Sources, in order:
+
+        1. Environment variable named `env_var`
+        2. Config file in app dir with name `file_name`
+        3. `default`
+    """
+    if env_value := os.environ.get(env_var):
+        return env_value
+
+    config_file = APP_DIR / file_name
+    if config_file.is_file():
+        return config_file.read_text()
+
+    return default