Agent assignment to a full cell leaves inconsistent state #3373
Description
Summary
When assigning a CellAgent to a cell that is already at capacity, an exception is raised, but the system is left in an
inconsistent state. After the failed assignment, agent.cell references the cell and the agent is not present in cell.agents. This violates the expected relationship between agents and cells.
Reproduction
from mesa.discrete_space.grid import OrthogonalVonNeumannGrid
from mesa.discrete_space.cell_agent import CellAgent
from mesa import Model
class DummyModel(Model):
pass
model = DummyModel()
grid = OrthogonalVonNeumannGrid((1, 1), capacity=1)
cell = grid[(0, 0)]
a1 = CellAgent(model)
a2 = CellAgent(model)
# Fill the cell
a1.cell = cell
# Attempt to overfill
try:
a2.cell = cell
except Exception as e:
print("Placement failed as expected:", e)
print("a2.cell:", a2.cell)
print("Is a2 in cell.agents?:", a2 in cell.agents)Current Behavior
The output is,
Placement failed as expected: ERROR: Cell is full
a2.cell: Cell((0, 0), [...])
Is a2 in cell.agents?: False
The agent believes it belongs to the cell, but the cell does not contain the agent.
Expected Behavior
If assignment fails due to capacity, the agent’s state should remain unchanged and consistent. The setter should not partially mutate state before raising.