If I can add to the conversation, do not use the samples argument in v4. The plan is to remove it, see Remove `samples` and `keep_size` from posterior_predictive · Issue #5775 · pymc-devs/pymc · GitHub. If you want to generate posterior predictive samples for a subset of the posterior only, you should provide a sliced posterior to sample_posterior_predictive as shown in the api docs example (bottom of the page): pymc.sample_posterior_predictive — PyMC 4.1.2 documentation
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