Document the test for the Snap.get method

tests/unit/test_snap.py

@@ -7,7 +7,7 @@
 import json
 import unittest
 from subprocess import CalledProcessError
-from typing import Iterable, Optional
+from typing import Any, Dict, Iterable, Optional
 from unittest.mock import MagicMock, mock_open, patch
 
 import fake_snapd as fake_snapd
@@ -842,7 +842,26 @@ def __getitem__(self, name: str) -> snap.Snap:
         self.assertIn("Failed to install or refresh snap(s): nothere", ctx.exception.message)
 
     def test_snap_get(self):
+        """Test the multiple different ways of calling the Snap.get function.
+
+        Valid ways:
+            ("key", typed=False) -> returns a string
+            ("key", typed=True) -> returns value parsed from json
+            (None, typed=True) -> returns parsed json for all keys
+            ("", typed=True) -> returns parsed json for all keys
+
+        An invalid key will raise an error if typed=False, but return None if typed=True.
+        """
         def fake_snap(command: str, optargs: Optional[Iterable[str]] = None) -> str:
+            """Snap._snap would normally call subprocess.check_output(["snap", ...], ...)
+
+            Here we only handle the "get" commands generated by Snap.get:
+                ["snap", "get", "-d"] -- equivalent to (None, typed=True)
+                ["snap", "get", "key"] -- equivalent to ("key", typed=False)
+                ["snap", "get", "-d" "key"] -- equivalent to ("key", typed=True)
+
+            Values are returned from the local keys_and_values dict instead of calling out to snap.
+            """
             assert command == "get"
             assert optargs is not None
             optargs = list(optargs)
@@ -862,7 +881,7 @@ def fake_snap(command: str, optargs: Optional[Iterable[str]] = None) -> str:
 
         foo = snap.Snap("foo", snap.SnapState.Latest, "stable", "1", "classic")
         foo._snap = MagicMock(side_effect=fake_snap)
-        keys_and_values = {
+        keys_and_values: Dict[str, Any] = {
             "key_w_string_value": "string",
             "key_w_float_value": 4.2,
             "key_w_int_value": 13,