Ensure grasspatches info is only collected if gras is true (projectme…

…sa#181)

the datacollector allways includes grasspatches even if grass is False. This is breaking (the proposed changes to agent storage in Model.

The underlying problem is that Model.get_agents_of_type now raises a KeyError if type does not exist. I think this is desirable behavior becuase errrors should not be passed over in silence. But this requires fixing the wolf sheep example as done here.

examples/wolf_sheep/wolf_sheep/model.py

@@ -79,15 +79,16 @@ def __init__(
         self.sheep_gain_from_food = sheep_gain_from_food
 
         self.grid = mesa.space.MultiGrid(self.width, self.height, torus=True)
-        self.datacollector = mesa.DataCollector(
-            {
-                "Wolves": lambda m: len(m.get_agents_of_type(Wolf)),
-                "Sheep": lambda m: len(m.get_agents_of_type(Sheep)),
-                "Grass": lambda m: len(
-                    m.get_agents_of_type(GrassPatch).select(lambda a: a.fully_grown)
-                ),
-            }
-        )
+
+        collectors = {
+            "Wolves": lambda m: len(m.get_agents_of_type(Wolf)),
+            "Sheep": lambda m: len(m.get_agents_of_type(Sheep)),
+        }
+
+        if grass:
+            collectors["Grass"] = lambda m: len(m.get_agents_of_type(GrassPatch))
+
+        self.datacollector = mesa.DataCollector(collectors)
 
         # Create sheep:
         for i in range(self.initial_sheep):