error when removing agents

Assume we have an agent that removes other agents from the simulation e.g. for simulating predator-prey I think I might have encountered a bug. When removing an agent from the model/environment in step 1, the method is called on the agent again in the next step which leads to an

AgentpyError
Agent 17 has no environment with topology '['grid', 'space', 'network']'

Here is a minimal example to reproduce the error:

import agentpy as ap

class SomeAgent(ap.Agent):

    def do_things(self):
        for n in self.neighbors():
            print(f"removing: {n}")
            n.delete()
            break

class SomeModel(ap.Model):
    def setup(self, **kwargs):
        self.grid = self.add_grid([self.p.size] * 2)
        self.grid.add_agents(self.p.agent_count, SomeAgent, random=True)

    def step(self):
        self.agents.do_things()


if __name__ == "__main__":
    params ={
        "agent_count": 100,
        "size": 20,
        "steps": 100
    }

    model = SomeModel(params)
    results = model.run()

I also tried to call the remove_agents function on the grid and model, but no luck.

Is this a bug, or am I doing it wrong?

Cheers

[JoelForamitti]

I added three features to address this issue in https://github.com/JoelForamitti/agentpy/pull/6:

First, there is a new method AgentList.call with an argument check_alive that can be used to prevent looping over deleted agents.

Second, Agent.neighbors can now also be called for agents without an environment, and will return an empty list instead of an error.

Third, there is a new property Agent.alive that can be used to check manually if an agent has been deleted.

The example from the beginning now works with the following adaption:

class SomeModel(ap.Model):
    def setup(self, **kwargs):
        self.grid = self.add_grid([self.p.size] * 2)
        self.grid.add_agents(self.p.agent_count, SomeAgent, random=True)

    def step(self):
        self.agents.call('do_things', check_alive=True)  # This is new

I tried to avoid to add a default check_alive to commands like self.agents.do_things() as it might slow models down that don’t do a lot of deletion. An issue for another time is that removing from lists is very inefficient, I’ve been thinking about using sets in the future.

[JoelForamitti]

That is a good idea! It would have to detect wether the element is still in the list agents. One way to do that could be to change the AttrList class from a list to an iterator over its parent list. I will try that out