Von Neumann grid hits TypeError on agents in corners

[maskarb]

Describe the bug

I am recreating the Forest Fire example with Mesa 2, and sometimes I hit the following traceback when the model iterates on an agent in the corner of the grid, specifically with Von Neumann neighborhood (when using Moore neighborhood, I haven't seen this exception).

(99, 99) <<< coordinates of agent when exception is raised
Uncaught exception GET /ws (127.0.0.1)
HTTPServerRequest(protocol='http', host='127.0.0.1:8521', method='GET', uri='/ws', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/.venv/lib/python3.11/site-packages/tornado/websocket.py", line 630, in _run_callback
    result = callback(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/.venv/lib/python3.11/site-packages/mesa_viz_tornado/ModularVisualization.py", line 230, in on_message
    self.application.model.step()
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/mesa_examples/forest_fire/forest_fire/model.py", line 50, in step
    self.schedule.step()
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/.venv/lib/python3.11/site-packages/mesa/time.py", line 128, in step
    self.do_each("step", shuffle=True)
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/.venv/lib/python3.11/site-packages/mesa/time.py", line 110, in do_each
    getattr(self._agents[agent_key], method)()
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/mesa_examples/forest_fire/forest_fire/agent.py", line 32, in step
    for neighbor in self.model.grid.iter_neighbors(self.pos, False):
  File "/Users/mskarbek/projects/opinion-dynamics-leaderboard/.venv/lib/python3.11/site-packages/mesa/space.py", line 401, in <genexpr>
    if (cell := self._grid[x][y]) != self.default_val()
                ~~~~~~~~~~^^^
TypeError: list indices must be integers or slices, not tuple

Expected behavior

I do not expect a traceback when stepping on agents in the corners of the grid.

To Reproduce

I've pushed the code to this repo:
https://github.com/maskarb/opinion-dynamics-leaderboard/pull/33/files

Using this seed: 8002739158563429809, you hit the error at step 24. In this case, the agent which triggers the traceback is in position (0, 99).

I just run the server:

$ mesa runserver

And then just start the run (keeping the default density).

Additional context

$ python --version                                                                                                        
Python 3.11.5

$ mesa version
mesa 2.1.2

[rht]

Thank you for the detailed report.

[rht]

Closed via #1832.