Reduces code complexity in one part of weather_symbols.py. (metoppv#1391

)

* Reduces code complexity in one part of weather_symbols.py.

* Refactors the dict definition so it is only performed once.

* Adds unit test for new error message.

* Removes == from list of invertible conditions, as it was wrong and isn't used anyway.

* Proves == raises the error too

* Improves doc-string

improver/wxcode/weather_symbols.py

@@ -60,6 +60,26 @@
 )
 
 
+def _define_invertible_conditions():
+    """Returns a dictionary of boolean comparator strings where the value is the
+    logical inverse of the key."""
+    invertible_conditions = {
+        ">=": "<",
+        ">": "<=",
+        "OR": "AND",
+        "": "",
+    }
+    # Add reverse {value: key} entries to invertible_conditions
+    reverse_inversions = {}
+    for k, v in invertible_conditions.items():
+        reverse_inversions[v] = k
+    invertible_conditions.update(reverse_inversions)
+    return invertible_conditions
+
+
+INVERTIBLE_CONDITIONS = _define_invertible_conditions()
+
+
 class WeatherSymbols(BasePlugin):
     """
     Definition and implementation of a weather symbol decision tree. This
@@ -228,12 +248,20 @@ def check_input_cubes(self, cubes):
         return optional_node_data_missing
 
     @staticmethod
-    def invert_condition(test_conditions):
+    def _invert_comparator(comparator):
+        """Inverts a single comparator string."""
+        try:
+            return INVERTIBLE_CONDITIONS[comparator]
+        except KeyError:
+            raise KeyError(f"Unexpected condition {comparator}, cannot invert it.")
+
+    def invert_condition(self, condition):
         """
-        Invert a comparison condition to select the negative case.
+        Invert a comparison condition to allow positive identification of conditions
+        satisfying the negative ('fail') case.
 
         Args:
-            test_conditions (dict):
+            condition (dict):
                 A single query from the decision tree.
         Returns:
             (tuple): tuple containing:
@@ -242,23 +270,10 @@ def invert_condition(test_conditions):
                 **inverted_combination** (str):
                     A string representing the inverted combination
         """
-        threshold = test_conditions["threshold_condition"]
-        inverted_threshold = threshold
-        if threshold == ">=":
-            inverted_threshold = "<"
-        elif threshold == "<=":
-            inverted_threshold = ">"
-        elif threshold == "<":
-            inverted_threshold = ">="
-        elif threshold == ">":
-            inverted_threshold = "<="
-        combination = test_conditions["condition_combination"]
-        inverted_combination = combination
-        if combination == "OR":
-            inverted_combination = "AND"
-        elif combination == "AND":
-            inverted_combination = "OR"
-
+        inverted_threshold = self._invert_comparator(condition["threshold_condition"])
+        inverted_combination = self._invert_comparator(
+            condition["condition_combination"]
+        )
         return inverted_threshold, inverted_combination
 
     @staticmethod

improver_tests/wxcode/wxcode/test_WeatherSymbols.py

@@ -381,6 +381,17 @@ def test_invert_combination_correctly(self):
             result = plugin.invert_condition(node)
             self.assertEqual(result[1], inverse_outputs[i])
 
+    def test_error(self):
+        """Test that the _invert_comparator method raises an error when the condition
+        cannot be inverted."""
+        plugin = WeatherSymbols()
+        possible_inputs = ["==", "!=", "NOT", "XOR"]
+        for val in possible_inputs:
+            with self.assertRaisesRegex(
+                KeyError, f"Unexpected condition {val}, cannot invert it."
+            ):
+                plugin._invert_comparator(val)
+
 
 class Test_construct_condition(IrisTest):