rapidsai/cudf

Audit pylibcudf for `noexcept`/`nogil` and remove "Exception check … requires the GIL" warnings

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#19,720 opened on Aug 18, 2025

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Description

Describe the bug When building pylibcudf, our CI logs show many performance warnings like:

performance hint: /__w/cudf/cudf/python/pylibcudf/pylibcudf/labeling.pyx:56:22:
Exception check after calling 'view' will always require the GIL to be acquired.
Declare 'view' as 'noexcept' if you control the definition and you're sure you
don't want the function to raise exceptions.

Example

https://github.com/rapidsai/cudf/actions/runs/17040978623/job/48305629162?pr=19446#step:12:330

Steps/Code to reproduce bug build pylibcudf with warnings

We should check pylibcudf for places where we’ve declared except * or except + but the C++ is non-throwing.

Expected behavior No perf warnings

Additional context Was recently fixed in RMM. See https://github.com/rapidsai/rmm/pull/2020

Contributor guide

Research direction
Start by building pylibcudf with warnings enabled to reproduce the issue (see CI logs linked). Examine the Cython .pyx files in pylibcudf for functions that have 'except *' or 'except +' declarations but call C++ functions that are marked noexcept. Compare with the approach in the RMM PR (rapidsai/rmm/pull/2020) which added noexcept to non throwing C++ functions. The goal is to add noexcept to all such C++ functions in cuDF called from Cython, and remove unnecessary except clauses in .pyx files.
Tech stack
cpppython
Domain
dataperformance
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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.
Clear
Prerequisites
C++ exception handlingCython syntaxBuilding pylibcudf with warnings
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45