time: Use weak references to agents in schedulers

[EwoutH]

This PR builts on the agents defaultdict introduced in #1894. See 89b61ec for the code changes proposed in this PR.

This PR modifies the schedulers to use weak references for agent management. The update aims to improve memory efficiency and garbage collection, especially in scenarios where agents are dynamically added and removed during model execution.

It also ensures that the Agent remove() method (introduced in #1894) also removes the Agent from the schedule.

Key changes

• Agents within schedulers are now stored as weak references (weakref.ref[Agent]), reducing the memory footprint and allowing for more efficient garbage collection.
• The add method in BaseScheduler and derived classes has been updated to store agents as weak references.
• The remove method has been modified to handle both direct agent objects and weak references, improving flexibility and robustness.
• Updated do_each method to iterate over a copy of agent keys and check the existence of agents before calling methods, ensuring stability during dynamic agent removal.
• Updated the agents property to return a list of live agent instances, maintaining backward compatibility for user code that iterates over agents.
• Ensured all existing tests pass, confirming the stability and correctness of the changes.

The changes result in a more memory-efficient scheduler implementation in Mesa, while preserving existing functionality and ensuring backward compatibility.

All 27 current tests pass without any modifications, so there shouldn't be changes breaking compatibility.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (9495a5a) 77.53% compared to head (26823e3) 77.47%.

Files	Patch %	Lines
mesa/time.py	86.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1910      +/-   ##
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- Coverage   77.53%   77.47%   -0.07%     
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  Files          15       15              
  Lines        1015     1021       +6     
  Branches      221      224       +3     
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+ Hits          787      791       +4     
  Misses        197      197              
- Partials       31       33       +2     

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[EwoutH]

Rebased on main, now that #1894 is merged.

[quaquel]

I think you can simply the code a bit by using WeakrefValueDictionary directly. This will make sure that items in the dict are automagically removed if referenced value no longer exists. So, some of the None checks you now have would become redundant because this is handled within the data structure for you.

[EwoutH]

Thanks! If we go this direction, I will certainly look into it.

Right now I would like to propose a different direction:

• Deprecate all schedulers and be free to do whatever you want #1912

Because this just looks way too good to ignore.

class MyModel(Model):
    def step(self):
        self.do_each("move")
        self.do_each("eat")
        self.do_each("sleep")

[quaquel]

My point was primarily about the fact that there are some useful collections in weakref that you want to use as much as possible. This point applies also when using these in spaces. It is always better to use a data structure that handles an issue (refs to None in this case) rather than solving it in your code (as you did in this draft).

The scheduler discussion is interesting and I am still thinking about it.

[EwoutH]

Fair point, thanks.

[quaquel]

How do you want to proceed with this now that #1916 is ready to be merged?

[EwoutH]

Probably close it. The next effort should be to refactor the schedulers using AgentSet functionality, and possibly deprecate them.

[quaquel]

Ok, if I get to it, I'll open a separate PR reimplementing the schedulers on top of AgentSet. I already had a quick look. It can be done in a backward compatible way while raising some warnings here and there.

[EwoutH]

Yeah it should be pretty easy, all the examples all already in #1916.

I have another deal: If you handle my email I will do this implementation 😇.

[EwoutH]

Closing this, succeeded by #1926.