Issue with data collection of PropertyLayer objects about mesa

[EwoutH]

Thanks for opening this issue and writing it up!

@rht I looked a bit into this issue and I can reproduce it. I think it has to do with either the model clock or the datacollector. Model._steps isn't updated for example with each step. It might be related to how #1942 was implemented. Do you have the opportunity to investigate this further?

[rht]

This is expected in the new API. Because you are not using model.schedule.step(), the model._steps doesn't get updated automatically. You need to manually advance the clock. See https://github.com/projectmesa/mesa-examples/blob/1e0d6b855c853c1dfa749348445a31000b4daca4/examples/schelling_experimental/model.py#L65-L77

[dylan-munson]

Thanks. I tried adding a call self_advance_time() just before data collection in the toy model and the problem persists. I should also note than in the model I was working with where this problem first arose, which is a version of the sugarscape_g1mt model from the mesa examples catalog, there is a call self._steps +=1 just before data collection, which I assume does essentially same thing. Thus, neither one seems to solve the problem.

[EwoutH]

I can reproduce this bug using the AgentSet API and other modern Mesa best practices.

import mesa

class testAgent(mesa.Agent):
    def __init__(self, unique_id, model):
        super().__init__(unique_id, model)
        self.pos: tuple
    def pollute(self):
        self.model.grid.properties["pollution"].modify_cell(self.pos, lambda p: p+1)

class testModel(mesa.Model):
    def __init__(self, width = 20, height = 20, initial_population=200):
        super().__init__()
        self.width = width
        self.height = height
        self.initial_population = initial_population
        
        # initiate mesa grid class
        self.grid = mesa.space.MultiGrid(self.width, self.height, torus=False)
        #add pollution property layer
        pollution_layer = mesa.space.PropertyLayer(name = "pollution", width = self.width, height = self.height, default_value = 0.0, dtype=float)
        self.grid.add_property_layer(pollution_layer)
        
        self.datacollector = mesa.DataCollector(model_reporters = {
            "Pollution": lambda m: m.grid.properties["pollution"].data,
            "Step": "_steps"
        })
        
        agent_id = 0
        for i in range(self.initial_population):
            # get agent position
            agent = testAgent(agent_id, self)
            self.grid.place_agent(agent, pos=mesa.model.random.choice(list(self.grid.empties)))
            agent_id += 1
            
    
    def step(self):
        self.agents.shuffle().do("pollute")

        print(f"Step {self._steps}: {self.grid.properties["pollution"].data[:1]}")
        self.datacollector.collect(self)
        self._advance_time()

# Run the model
model = testModel()
for i in range(20):
    model.step()

model_results = model.datacollector.get_model_vars_dataframe()
model_results

It prints the correct values each step, so it seems the bug is really in the datacollector itself.

[EwoutH]

I suspect for some reason it doesn't save a copy of the values at that timestep to model_vars, but a reference to the values that gets continuously updated with each step. @rht do you have an idea where that could happen?

[EwoutH]

Found the issue and implemented a fix in #2129.