Fix handling of seasonal periods that straddle years (fixes #226)

climix/period.py

@@ -55,17 +55,29 @@ class Season(Period):
     ]
 
     def __init__(self, specifier):
-        super().__init__("year", "sem")
+        super().__init__("season_year", "sem")
         self.specifier = specifier.upper()
         self.first_month_number = (self.YEAR * 2).find(self.specifier) + 1
         self.length = len(self.specifier)
-        self.last_month_number = self.first_month_number + self.length - 1
-        self.constraint = iris.Constraint(
-            time=lambda cell: self.first_month_number
-            <= cell.point.month
-            <= self.last_month_number
+        last_month_number = self.first_month_number + self.length - 1
+        if last_month_number > 12:
+            last_month_number %= 12
+
+            def selector(cell):
+                m = cell.point.month
+                return (self.first_month_number <= m <= 12) | (
+                    1 <= m <= last_month_number
                 )
 
+        else:
+
+            def selector(cell):
+                m = cell.point.month
+                return self.first_month_number <= m <= last_month_number
+
+        self.constraint = iris.Constraint(time=selector)
+        self.last_month_number = last_month_number
+
     def long_label(self):
         first_month = self.MONTHS[self.first_month_number - 1]
         last_month = self.MONTHS[self.last_month_number - 1]
@@ -73,7 +85,7 @@ class Season(Period):
         return long_label
 
     def add_coord_categorisation(self, cube):
-        iris.coord_categorisation.add_year(
+        iris.coord_categorisation.add_season_year(
             cube, self.input_coord, name=self.output_coord
         )
         return self.output_coord