From 373637ed28566f0c35832bfd5b51d44eae8739ab Mon Sep 17 00:00:00 2001
From: Jangsea Park <jangsea.park@gmail.com>
Date: Wed, 26 Oct 2022 17:08:30 +0900
Subject: [PATCH] Implement swap_pos

---
 mesa/space.py      | 20 ++++++++++++++++++++
 tests/test_grid.py | 28 ++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/mesa/space.py b/mesa/space.py
index 16da1fbd7b5..362099671a2 100644
--- a/mesa/space.py
+++ b/mesa/space.py
@@ -507,6 +507,26 @@ 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)
diff --git a/tests/test_grid.py b/tests/test_grid.py
index 3b43872ee43..3104ba2e68e 100644
--- a/tests/test_grid.py
+++ b/tests/test_grid.py
@@ -270,6 +270,34 @@ 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