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Fix TypeError in find_combinations when evaluation_func is None#3112

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tpike3 merged 2 commits intomesa:mainfrom
codebyNJ:fix/meta-agent-evaluate-combination-none-handling
Jan 19, 2026
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Fix TypeError in find_combinations when evaluation_func is None#3112
tpike3 merged 2 commits intomesa:mainfrom
codebyNJ:fix/meta-agent-evaluate-combination-none-handling

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@codebyNJ codebyNJ commented Jan 11, 2026

Summary

Fixed a TypeError that occurs when calling find_combinations() without an evaluation_func parameter.

Bug / Issue

In meta_agent.py, the find_combinations() function calls evaluate_combination() which returns None when evaluation_func is None. The code then attempts to unpack this None value:

group_set, result = evaluate_combination(candidate_group, model, evaluation_func)

This causes: TypeError: cannot unpack non-iterable NoneType object

Implementation

File changed: mesa_agent.py

Changed From:

group_set, result = evaluate_combination(candidate_group, model, evaluation_func)
if result:
    combinations.append((group_set, result))

To:

evaluation_result = evaluate_combination(candidate_group, model, evaluation_func)
if evaluation_result is not None:
    group_set, result = evaluation_result
    if result:
        combinations.append((group_set, result))

Testing

  • Added test test_find_combinations_without_evaluation_func that validates the fix
  • All 16 meta_agents tests pass
  • ruff checks pass

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github-actions bot commented Jan 11, 2026

Performance benchmarks:

Model Size Init time [95% CI] Run time [95% CI]
BoltzmannWealth small 🔵 +1.8% [+0.8%, +2.8%] 🔵 +0.1% [+0.0%, +0.2%]
BoltzmannWealth large 🔵 +0.5% [-0.4%, +1.2%] 🔴 +5.2% [+3.7%, +6.8%]
Schelling small 🔵 +1.0% [-0.1%, +2.1%] 🔵 +0.0% [-0.5%, +0.6%]
Schelling large 🔵 +1.2% [-0.4%, +3.9%] 🔵 +0.8% [-0.7%, +2.3%]
WolfSheep small 🔵 -0.5% [-0.8%, -0.3%] 🔵 -1.8% [-1.9%, -1.6%]
WolfSheep large 🔵 -1.4% [-7.0%, +4.6%] 🔵 -4.0% [-5.9%, -2.0%]
BoidFlockers small 🔵 +1.5% [+1.2%, +1.8%] 🔵 +1.3% [+1.2%, +1.5%]
BoidFlockers large 🔵 +2.0% [+1.4%, +2.6%] 🔵 +0.5% [+0.3%, +0.7%]

@quaquel
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quaquel commented Jan 11, 2026

I am not sure this is the right fix. The entire function is pointless if evaluation_func is None. So, my guess is that either evaluation_func should become an argument or calling it with None should raise a ValueError. @tpike3, can you clarify your intent here?

@EwoutH EwoutH requested a review from tpike3 January 17, 2026 22:08
tpike3
tpike3 approved these changes Jan 19, 2026
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@codebyNJ Thanks for the catch and fix!

@tpike3 tpike3 merged commit ea1c165 into mesa:main Jan 19, 2026
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@EwoutH
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EwoutH commented Jan 19, 2026

I am not sure this is the right fix. The entire function is pointless if evaluation_func is None. So, my guess is that either evaluation_func should become an argument or calling it with None should raise a ValueError. @tpike3, can you clarify your intent here?

I would still be interested in this.

@EwoutH EwoutH added the bug Release notes label label Feb 11, 2026
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