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

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

Tech stack
python
Domain
backenddeveloper experience
Issue type
refactor
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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
basic Pythonfamiliarity with PyMC
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the conditions that trigger the assert checks at pymc/logprob/scan.py lines 115-116. Replace each assert with a meaningful error message (e.g., using raise TypeError or ValueError) that explains the issue and suggests possible fixes. Test the changes by running existing tests and considering edge cases with multiple scan outputs.