pymc-devs / pymc-devs/pytensor

Scan NIT-SOT with 0 steps have wrong shape

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bug Op implementation scan
Dominant language
Python
Stars
644
Forks
208
Avg merge
2d 14h
Merged PRs (30d)
16

Description

Description
import pytensor
import pytensor.tensor as pt

x0 = pt.vector("x0")
n = pt.iscalar("n")
outs, _ = pytensor.scan(lambda xtm1: (xtm1 + 1, xtm1), outputs_info=[x0, None], n_steps=n)
fn = pytensor.function([n, x0], outs)
fn(n=0, x0=[1, 2, 3])
# [array([], shape=(0, 3), dtype=float64),
#  array([], shape=(0, 0), dtype=float64)]

Unless the nitsot have static output shape I don't think we can figure out the correct shape without evaluating the function atleast once. We should raise or add some extra logic to handle that case.

Originally Scan didn't allow 0 steps, that was relaxed in https://github.com/aesara-devs/aesara/pull/741

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Research direction

Reproduce the reported case using pytensor.scan with n_steps=0 and inspect the scan shape-inference path that handles the NIT-SOT output. Determine whether the intended behavior is to infer the shape or raise an error, then add a regression test covering the zero-step result and verify the reported output shapes.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
backend
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
38/100

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