AgentSet: Add agg method (#2266)

This PR introduces the `agg` method to the `AgentSet` class, allowing users to apply aggregation functions (e.g., `min`, `max`, `sum`, `np.mean`) to attributes of agents within the `AgentSet`. This enhancement makes it easier to compute summary statistics across agent attributes directly within the `AgentSet` interface.

This will be useful in both the model operation itself as well as for future DataCollector use.

mesa/agent.py

@@ -304,6 +304,20 @@ def map(self, method: str | Callable, *args, **kwargs) -> list[Any]:
 
         return res
 
+    def agg(self, attribute: str, func: Callable) -> Any:
+        """
+        Aggregate an attribute of all agents in the AgentSet using a specified function.
+
+        Args:
+            attribute (str): The name of the attribute to aggregate.
+            func (Callable): The function to apply to the attribute values (e.g., min, max, sum, np.mean).
+
+        Returns:
+            Any: The result of applying the function to the attribute values. Often a single value.
+        """
+        values = self.get(attribute)
+        return func(values)
+
     def get(self, attr_names: str | list[str]) -> list[Any]:
         """
         Retrieve the specified attribute(s) from each agent in the AgentSet.

tests/test_agent.py

@@ -1,5 +1,6 @@
 import pickle
 
+import numpy as np
 import pytest
 
 from mesa.agent import Agent, AgentSet
@@ -276,6 +277,41 @@ def remove_function(agent):
     assert len(agentset) == 0
 
 
+def test_agentset_agg():
+    model = Model()
+    agents = [TestAgent(i, model) for i in range(10)]
+
+    # Assign some values to attributes
+    for i, agent in enumerate(agents):
+        agent.energy = i + 1
+        agent.wealth = 10 * (i + 1)
+
+    agentset = AgentSet(agents, model)
+
+    # Test min aggregation
+    min_energy = agentset.agg("energy", min)
+    assert min_energy == 1
+
+    # Test max aggregation
+    max_energy = agentset.agg("energy", max)
+    assert max_energy == 10
+
+    # Test sum aggregation
+    total_energy = agentset.agg("energy", sum)
+    assert total_energy == sum(range(1, 11))
+
+    # Test mean aggregation using numpy
+    avg_wealth = agentset.agg("wealth", np.mean)
+    assert avg_wealth == 55.0
+
+    # Test aggregation with a custom function
+    def custom_func(values):
+        return sum(values) / len(values)
+
+    custom_avg_energy = agentset.agg("energy", custom_func)
+    assert custom_avg_energy == 5.5
+
+
 def test_agentset_set_method():
     # Initialize the model and agents with and without existing attributes
     class TestAgentWithAttribute(Agent):