bug in ContinuousSpace.get_neighbors

[quaquel]

Describe the bug
Currently, get_neighbors will fail to identify neighbors if two agents occupy the exact same position.

The line that causes the issue is shown below. if include_center is True, this will return all agents on the position. If if include_center is False, it will return no agents. However, in case of 2 agents on the same location, neither is correct.

        neighbors = [
            self._index_to_agent[x] for x in idxs if include_center or dists[x] > 0
        ]

Expected behavior
Return any neighbors that are not the agent for which you are calling get_neighbors.

To Reproduce
Create a space with 2 agents in the same location and call get_neighbors with the position of either agent.

model = Model(seed=42)
space = ContinuousSpace(1, 1, torus=False)
agent1 = Agent(model)
space.place_agent(agent1, (0.5, 0.5))

agent2 = Agent(model)
space.place_agent(agent2, (0.5, 0.5))

# returns no agents, should return agent2
space.get_neighbors(agent1.pos, radius=1, include_center=False)

# returns both agents, should return only agent2
space.get_neighbors(agent1.pos, radius=1, include_center=True)

[Sahil-Chhoker]

Upon looking at code I found that it is impossible to find the agent which is passed in the method, because you are passing the position, and since every agent has the same position, if you try to remove the so called current agent, you will have to remove a random one or none at all.

To maybe overcome this one can change the type of the pos variable, that is if an agent is passed in the method, we can track that agent and exclude it from the result.

Can something like this work?

if isinstance(pos, Agent):
            self.current_agent = pos
            pos = pos.pos
        else:
            self.current_agent = None

...

neighbors = [
            self._index_to_agent[x] for x in idxs
        ]

if not self.current_agent:
    return neighbors
else:
    neighbors.remove(self.current_agent)
    return neighbors

[quaquel]

You are absolutely right that this bug cannot be fixed inside the method as it currently stands. However, changing the method signature is a breaking change and could have a profound impact on existing models. So, that is not an ideal solution.

Practically speaking, with #2584 almost ready to go, I am thinking of the following

1. explicitly document the current buggy behavior. This is the minimum thing to do.
2. Add a new method with a different signature with the correct behavior. Not sure about this in light of Reimplementation of Continuous Space #2584.
3. Update the boid flocking example to use Reimplementation of Continuous Space #2584, the new method, or work around the bug. The latter is, again, a minimum thing to do.

[EwoutH]

# returns no agents, should return agent2
space.get_neighbors(agent1.pos, radius=1, include_center

I don't think I agree this is unintended behavior. The argument is called include_center, not include_self. By setting it (the former) False, it does exactly what you expect: Return neighbors not at the same location as you.

That said, it might be helpful to have an include/exclude self argument. We could add that to the new API.

[quaquel]

I don't think I agree this is unintended behavior.

Let's agree to disagree. The documentation just clarifies the behavior even more.

That said, it might be helpful to have an include/exclude self argument. We could add that to the new API.

The problem cannot occur there because I draw a sharp distinction between getting agents from a space and getting neighbors from an agent.