pymc-devs/pymc

Replace `assert` checks in `logprob.scan` with real error handling

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#7,780 opened on May 9, 2025

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enhancementshackathonhelp wantedneeds infopytensor

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Description

Description

There are two annoying assert checks that I commonly run up against when writing time series models here and here. Other users are hitting them too, see for example here.

It would be much nicer to have a real error message that explains what has gone wrong and why, or at least gives some information about which parts of the graph are going wrong.

I'm not 100% sure how one ends up hitting these asserts, so I tagged this with needs info. It seems to have to do with having multiple outputs from the scan inner function in a custom dist.

Contributor guide

Research direction
Examine the assert statements in pymc/logprob/scan.py lines 115-116, understand the possible graph scenarios that trigger them, and craft descriptive error messages that explain the issue and suggest corrective actions.
Tech stack
python
Domain
backend
Issue type
Refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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.
Mostly clear
Prerequisites
Pythonpymc
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65