pymc-devs / pymc-devs/pytensor

Scan invalid indexing is not always caught

Open
#215 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

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

Description

Description
import numpy as np
import pytensor
import pytensor.tensor as pt

print("Testing with n_steps")
x = pt.scalar("x")
for n_steps in (0, 2, 100, 1):
    ys, _, = pytensor.scan(
        lambda xtm1: xtm1 + 1,
        outputs_info=[x],
        n_steps=n_steps,
    )

    for test in (200, -200):
        try:
            ys[test].eval({x: 0})
        except IndexError:
            pass
        else:
            print(f"ys[{test}] did not raise for {n_steps=}")
            
print("Testing with sequences")
xs = pt.vector("xs")
for n_steps in (0, 2, 100, 1):
    zs, _, = pytensor.scan(
        lambda o, x: o + x,
        outputs_info=[pt.zeros(())],
        sequences=[xs]
    )

    for test in (200, -200):
        try:
            zs[test].eval({xs: np.arange(n_steps)})
        except IndexError:
            pass
        except ValueError:
            print(f"zs[{test}] failed with a wrong ValueError for {n_steps=}")
        else:
            print(f"zs[{test}] did not raise for {n_steps=}")
Testing with n_steps
ys[-200] did not raise for n_steps=2
ys[-200] did not raise for n_steps=100
ys[200] did not raise for n_steps=1
ys[-200] did not raise for n_steps=1
Testing with sequences
zs[200] failed with a wrong ValueError for n_steps=0
zs[-200] failed with a wrong ValueError for n_steps=0
zs[-200] did not raise for n_steps=2
zs[-200] did not raise for n_steps=100
zs[-200] did not raise for n_steps=1

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the examples using pytensor.scan, the ys/zs indexing expressions, and eval calls shown in the issue. Trace how out-of-range positive and negative indexes are handled for n_steps and sequences. Done means invalid indexes consistently raise IndexError instead of being silently accepted or producing the reported ValueError.

Written by the indexing model from the issue text.

Assessment

Tech stack
numpy, python
Domain
backend
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
42/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.