clamp season to years with data

If there is no data for the given season, use data from the closest
season instead.

src/geo/populations.py

@@ -224,8 +224,10 @@
   },
 }
 
+first_season, last_season = min(population_weights), max(population_weights)
 
-def get_population_weight(location, season=None):
+
+def get_population_weight(location, season=last_season):
   """
   Return the population weight of the given location, relative to the US
   nationally.
@@ -243,12 +245,11 @@ def get_population_weight(location, season=None):
   output:
     - the fraction of the US population contained within the given location
   """
-  if not season:
-    season = max(population_weights)
+  season = max(min(season, last_season), first_season)
   return population_weights[season][location]
 
 
-def get_population(location, season=None):
+def get_population(location, season=last_season):
   """
   Return the approximate population of the given location. The returned value
   is rounded to the nearest integer and is based on the assumption that the US

tests/geo/test_populations.py

@@ -38,6 +38,20 @@ def test_season_defaults_to_latest(self):
     val2 = get_population_weight(location)
     self.assertEqual(val1, val2)
 
+  def test_season_clamped_to_first(self):
+    location = 'nc'
+    season = min(population_weights)
+    val1 = get_population_weight(location, season)
+    val2 = get_population_weight(location, season - 1)
+    self.assertEqual(val1, val2)
+
+  def test_season_clamped_to_last(self):
+    location = 'sc'
+    season = max(population_weights)
+    val1 = get_population_weight(location, season)
+    val2 = get_population_weight(location, season + 1)
+    self.assertEqual(val1, val2)
+
   def test_population_is_integer(self):
     pop = get_population('vi')
     self.assertTrue(isinstance(pop, int))