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Fix #2987: IndexError in batch_run with sparse data collection #2988

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Fix #2987: IndexError in batch_run with sparse data collection #2988

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@Nithin9585 Nithin9585 commented Dec 22, 2025

Fixes #2987

This PR resolves an IndexError that occurs in batch_run() when models use sparse data collection (collecting data only at specific steps rather than every step).

Problem

The _collect_data() function in batchrunner.py incorrectly used step numbers as list indices:

model_data = {param: values[step] for param, values in dc.model_vars.items()}

When a model collects data sparsely (e.g., only at steps 0, 5, 10), the model_vars list has only 3 items (indices 0, 1, 2). Attempting to access values[5] or values[10] causes an IndexError.

Solution

Changed the implementation to map step numbers to their corresponding collection indices:

available_steps = sorted(dc._agent_records.keys())
if step not in available_steps:
    step = max((s for s in available_steps if s <= step), default=0)

try:
    collection_index = available_steps.index(step)
except ValueError:
    collection_index = 0

model_data = {param: values[collection_index] for param, values in dc.model_vars.items()}

Testing

Added test_batch_run_sparse_collection() to verify the fix works with models that collect data every N steps.

Test output:

tests/test_batch_run.py::test_batch_run_sparse_collection PASSED [100%]

Changes

  • mesa/batchrunner.py: Fixed the IndexError in _collect_data()
  • tests/test_batch_run.py: Added regression test for sparse data collection

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github-actions bot commented Dec 22, 2025

Performance benchmarks:

Model Size Init time [95% CI] Run time [95% CI]
BoltzmannWealth small 🔵 +0.8% [+0.4%, +1.3%] 🔵 -1.0% [-1.2%, -0.9%]
BoltzmannWealth large 🔵 +1.3% [-0.6%, +4.6%] 🔵 -1.0% [-2.9%, +0.5%]
Schelling small 🔵 -0.3% [-1.3%, +0.7%] 🔵 -1.1% [-1.7%, -0.4%]
Schelling large 🔵 -0.3% [-1.7%, +1.8%] 🔵 -1.4% [-2.3%, -0.6%]
WolfSheep small 🔵 +0.2% [-0.6%, +0.8%] 🔵 -0.4% [-0.8%, +0.0%]
WolfSheep large 🔵 -1.1% [-4.6%, +1.2%] 🔵 -0.2% [-0.9%, +0.5%]
BoidFlockers small 🔵 -0.7% [-1.0%, -0.5%] 🔵 +0.0% [-0.2%, +0.3%]
BoidFlockers large 🔵 -1.2% [-1.9%, -0.5%] 🔵 -0.3% [-0.7%, +0.1%]

@Nithin9585 Nithin9585 force-pushed the fix-batchrunner-sparse-collection branch from ee4f9b5 to 43b87aa Compare December 22, 2025 11:40
@falloficarus22
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falloficarus22 commented Dec 22, 2025 •
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Hey, isn't it better to simply use values[-1] to get the latest collected data instead of trying to map steps to indices. This would be safer and work for all cases.

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Nithin9585 commented Dec 23, 2025

@falloficarus22 Good point! But if data exists at steps [0, 5, 10] and we request step 3, wouldn't values[-1] return step 10's data when we actually want step 0's?

How would your approach handle different requested steps?

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falloficarus22 commented Dec 23, 2025

@falloficarus22 Good point! But if data exists at steps [0, 5, 10] and we request step 3, wouldn't values[-1] return step 10's data when we actually want step 0's?

How would your approach handle different requested steps?

Well I thought it was possible to do with values[-1] and tried to apply the changes locally and I'm running into multiple errors.
So, I guess this approach is fine.

Your pre-commit is failing (maybe you didn't ruff check and ruff format the code).

@Nithin9585 Nithin9585 force-pushed the fix-batchrunner-sparse-collection branch from e69551a to dae37fe Compare December 23, 2025 10:25
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Nithin9585 commented Dec 23, 2025

@quaquel @EwoutH can you please review this pr ?

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quaquel commented Jan 5, 2026

I tried to resolve the conflicts with main, but failed. Tests are falling at the moment. Do you have time to investigate?

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BatchRunner crashes with sparse data collection (IndexError)

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@Nithin9585 @falloficarus22 @quaquel