pymc-devs/pymc

`TestPopulationSamplers` started failing in the CI

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#8,169 opened on Mar 5, 2026

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Description

Description

FAILED tests/sampling/test_population.py::TestPopulationSamplers::test_checks_population_size - ValueError: setting an array element with a sequence.
FAILED tests/sampling/test_population.py::TestPopulationSamplers::test_demcmc_warning_on_small_populations - ValueError: setting an array element with a sequence.
FAILED tests/sampling/test_population.py::TestPopulationSamplers::test_nonparallelized_chains_are_random - ValueError: setting an array element with a sequence.
FAILED tests/sampling/test_population.py::TestPopulationSamplers::test_parallelized_chains_are_random - ValueError: setting an array element with a sequence.

Contributor guide

Research direction
Investigate the ValueError occurring in population sampling tests. The error 'setting an array element with a sequence' suggests a shape mismatch when constructing arrays. Trace the test logic and check recent changes to population samplers.
Tech stack
python
Domain
datamachine learning
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Needs investigation
Prerequisites
Python
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50