fix(Keithley3706A): Fix interlock status dictionary

docs/changes/newsfragments/5013.improved_driver

@@ -0,0 +1,6 @@
+Fixed a bug in interlock status querying for Keithley 3706A. Originally, not all
+potential responses from the system were accounted for when querying for interlock
+status. A dictionary is used to map the response from the system to a string describing
+the interlock status. When the system returns a response that was not accounted for, this
+resulted in a KeyError being raised. Now, this dictionary accounts for all potential responses
+from the system.

qcodes/instrument/sims/Keithley_3706A.yaml

@@ -68,7 +68,7 @@ devices:
       - q: "print(slot[6].idn)"
         r: "Empty Slot"
       - q: "print(slot[1].interlock.state)"
-        r: 0
+        r: "nil"
         type: int
       - q: "print(slot[2].interlock.state)"
         r: 0

qcodes/instrument_drivers/Keithley/Keithley_3706A.py

@@ -793,15 +793,28 @@ def get_interlock_state(self) -> Tuple[Dict[str, str], ...]:
         cannot be energized.
         """
         slot_id = self._get_slot_ids()
-        interlock_status = {0: "Interlock is disengaged", 1: "Interlock is engaged"}
+        interlock_status = {
+            None: (
+                "No card is installed or the installed card does "
+                "not support interlocks"
+            ),
+            0: "Interlocks 1 and 2 are disengaged on the card",
+            1: "Interlock 1 is engaged, interlock 2 (if it exists) is disengaged",
+            2: "Interlock 2 in engaged, interlock 1 is disengaged",
+            3: "Both interlock 1 and 2 are engaged",
+        }
         states: List[Dict[str, str]] = []
         for i in slot_id:
             state = self.get_interlock_state_by_slot(i)
             states.append({"slot_no": i, "state": interlock_status[state]})
         return tuple(states)
 
-    def get_interlock_state_by_slot(self, slot: Union[str, int]) -> int:
-        return int(float(self.ask(f"slot[{int(slot)}].interlock.state")))
+    def get_interlock_state_by_slot(self, slot: Union[str, int]) -> Union[int, None]:
+        state = self.ask(f"slot[{int(slot)}].interlock.state")
+        if state == "nil":
+            return None
+        else:
+            return int(float(state))
 
     def get_ip_address(self) -> str:
         """

qcodes/tests/drivers/test_keithley_3706A.py

@@ -80,9 +80,18 @@ def test_slot_names(driver):
 
 def test_get_interlock_state(driver):
     dict_list = (
-        {'slot_no': '1', 'state': 'Interlock is disengaged'},
-        {'slot_no': '2', 'state': 'Interlock is disengaged'},
-        {'slot_no': '3', 'state': 'Interlock is engaged'},
+        {
+            "slot_no": "1",
+            "state": (
+                "No card is installed or the installed card does "
+                "not support interlocks"
+            ),
+        },
+        {"slot_no": "2", "state": "Interlocks 1 and 2 are disengaged on the card"},
+        {
+            "slot_no": "3",
+            "state": "Interlock 1 is engaged, interlock 2 (if it exists) is disengaged",
+        },
     )
     assert dict_list == driver.get_interlock_state()