Make swap_pos part of Grid instead of SingleGrid (projectmesa#1507)

* Make swap_pos part of Grid instead of SingleGrid

* Update test_grid.py

The second commit changes the agents sampling to
`agent_a, agent_b = list(filter(None, self.grid))[:2]`
because when the test is moved to `TestBaseGrid`, the `self.grid` in `TestBaseGrid` is not full.

mesa/space.py

@@ -408,6 +408,26 @@ def remove_agent(self, agent: Agent) -> None:
         self.empties.add(pos)
         agent.pos = None
 
+    def swap_pos(self, agent_a: Agent, agent_b: Agent) -> None:
+        """Swap agents positions"""
+        agents_no_pos = []
+        if (pos_a := agent_a.pos) is None:
+            agents_no_pos.append(agent_a)
+        if (pos_b := agent_b.pos) is None:
+            agents_no_pos.append(agent_b)
+        if agents_no_pos:
+            agents_no_pos = [f"<Agent id: {a.unique_id}>" for a in agents_no_pos]
+            raise Exception(f"{', '.join(agents_no_pos)} - not on the grid")
+
+        if pos_a == pos_b:
+            return
+
+        self.remove_agent(agent_a)
+        self.remove_agent(agent_b)
+
+        self.place_agent(agent_a, pos_b)
+        self.place_agent(agent_b, pos_a)
+
     def is_cell_empty(self, pos: Coordinate) -> bool:
         """Returns a bool of the contents of a cell."""
         x, y = pos
@@ -492,26 +512,6 @@ def position_agent(
             coords = (x, y)
             self.place_agent(agent, coords)
 
-    def swap_pos(self, agent_a: Agent, agent_b: Agent) -> None:
-        """Swap agents positions"""
-        agents_no_pos = []
-        if (pos_a := agent_a.pos) is None:
-            agents_no_pos.append(agent_a)
-        if (pos_b := agent_b.pos) is None:
-            agents_no_pos.append(agent_b)
-        if agents_no_pos:
-            agents_no_pos = [f"<Agent id: {a.unique_id}>" for a in agents_no_pos]
-            raise Exception(f"{', '.join(agents_no_pos)} - not on the grid")
-
-        if pos_a == pos_b:
-            return
-
-        self.remove_agent(agent_a)
-        self.remove_agent(agent_b)
-
-        self.place_agent(agent_a, pos_b)
-        self.place_agent(agent_b, pos_a)
-
     def place_agent(self, agent: Agent, pos: Coordinate) -> None:
         if self.is_cell_empty(pos):
             super().place_agent(agent, pos)

tests/test_grid.py

@@ -169,6 +169,36 @@ def test_agent_remove(self):
         assert agent.pos is None
         assert self.grid.grid[x][y] is None
 
+    def test_swap_pos(self):
+
+        # Swap agents positions
+        agent_a, agent_b = list(filter(None, self.grid))[:2]
+        pos_a = agent_a.pos
+        pos_b = agent_b.pos
+
+        self.grid.swap_pos(agent_a, agent_b)
+
+        assert agent_a.pos == pos_b
+        assert agent_b.pos == pos_a
+        assert self.grid[pos_a] == agent_b
+        assert self.grid[pos_b] == agent_a
+
+        # Swap the same agents
+        self.grid.swap_pos(agent_a, agent_a)
+
+        assert agent_a.pos == pos_b
+        assert self.grid[pos_b] == agent_a
+
+        # Raise for agents not on the grid
+        self.grid.remove_agent(agent_a)
+        self.grid.remove_agent(agent_b)
+
+        id_a = agent_a.unique_id
+        id_b = agent_b.unique_id
+        e_message = f"<Agent id: {id_a}>, <Agent id: {id_b}> - not on the grid"
+        with self.assertRaisesRegex(Exception, e_message):
+            self.grid.swap_pos(agent_a, agent_b)
+
 
 class TestBaseGridTorus(TestBaseGrid):
     """
@@ -268,34 +298,6 @@ def test_enforcement(self, mock_model):
         with self.assertRaises(Exception):
             self.move_to_empty(self.agents[0], num_agents=self.num_agents)
 
-        # Swap agents positions
-        agent_a, agent_b = random.sample(list(self.grid), k=2)
-        pos_a = agent_a.pos
-        pos_b = agent_b.pos
-
-        self.grid.swap_pos(agent_a, agent_b)
-
-        assert agent_a.pos == pos_b
-        assert agent_b.pos == pos_a
-        assert self.grid[pos_a] == agent_b
-        assert self.grid[pos_b] == agent_a
-
-        # Swap the same agents
-        self.grid.swap_pos(agent_a, agent_a)
-
-        assert agent_a.pos == pos_b
-        assert self.grid[pos_b] == agent_a
-
-        # Raise for agents not on the grid
-        self.grid.remove_agent(agent_a)
-        self.grid.remove_agent(agent_b)
-
-        id_a = agent_a.unique_id
-        id_b = agent_b.unique_id
-        e_message = f"<Agent id: {id_a}>, <Agent id: {id_b}> - not on the grid"
-        with self.assertRaisesRegex(Exception, e_message):
-            self.grid.swap_pos(agent_a, agent_b)
-
 
 # Number of agents at each position for testing
 # Initial agent positions for testing