[sdiebolt]

Let Python threads run during ITK pipeline updates, and make the regression test check filtering rather than setup. Closes #7000.

Why the GIL stayed locked

The 2019 GIL-release change enabled SWIG threading but also set nothreadallow, citing this SWIG discussion. That advice served a different goal: protecting director callbacks while deliberately avoiding GIL release around native calls. The combination therefore did not provide the concurrency described by the commit.

The later -threads addition in #5776/#6022 left that override in place. Its regression test accepted overlapping setup/filtering intervals as evidence of GIL release. Measured intervals were about 0.81 seconds, but only 0.002–0.003 seconds were inside Update().

What this fixes

• Release the GIL for Update() and UpdateLargestPossibleRegion(), retaining it for unrelated wrappers and respecting ITK_PYTHON_RELEASE_GIL.
• Reacquire the GIL in PyImageFilter callbacks and before PyCommand validates observer callables.
• Replace the overlap test with a waiting-thread probe around preconstructed filters, suppressing periodic Python thread switching.
• Exercise native Update() exceptions instead of errors that could arise in Python-side file-reading setup.
• Document the scope and select an available wrapped type for the Python-filter test.

Validation

Linux x86_64, CPython 3.13.9, NumPy 2.3.4, Release build:

• Unpatched wrappers: original regression passed; replacement failed.
• Patched wrappers: replacement passed 20 consecutive runs; all three targeted tests passed.
• Issue Python Update() appears to hold the GIL despite GIL-release support #7000's ticker workload: 913–927 ticks during 0.97–0.98 seconds of filtering.
• Compiled SWIG probes confirmed enabled/disabled threading behavior; local hooks passed.
• An instrumented observer probe detected PyCallable_Check() without the GIL before the PyCommand fix; the probe and full safety script pass after it.

The initial Linux CI run also passed all three targeted tests. Its separate median type-hint failure is described in the PR comments.

Full local suite, Python 3.14 wheel, and full GIL-disabled native build validation were not run. CI must validate the latest revision.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any) — no new files
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5 — no new classes
• Added ITK examples for all new major features (if any) — bug fix, no new major feature

AI assistance

AI assistance supported diagnosis, implementation, and local testing.

[greptile-apps]

Confidence Score: 4/5

[High impact] The change affects pipeline updates across ITK’s Python wrappers.

Fix the unchecked float requirement in PyImageFilterTest before merging.

Findings

1.  Float-disabled builds fail this test ▶

Summary

The PR enables GIL release for Update() and UpdateLargestPossibleRegion() and adds GIL guards around Python filter callbacks.

• ITK pipeline updates let other Python threads run.
• Python filter callbacks run with the GIL held.

Diagram

sequenceDiagram
  participant P as Python thread
  participant W as Update wrapper
  participant F as Native pipeline
  participant C as Python callback
  P->>W: Update()
  W->>W: Release GIL when enabled
  W->>F: Run pipeline
  F->>C: Acquire GIL and call Python
  C-->>F: Return and release acquired GIL
  F-->>W: Finish update
  W->>W: Reacquire GIL
  W-->>P: Return or raise

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_{Reviews (1) · Last reviewed commit: "BUG: Release the Python GIL during pipel..." · Reviewed by Greptile}

[sdiebolt]

The Linux failure looks separate from this fix: filtering succeeds, then the median test’s type-hint check raises NameError: name 'xr' is not defined. The unchanged parent-commit type alias reproduces it when xarray is available.

All three GIL-related tests passed in that job. The float-wrapping review finding is fixed in 2d8272e; those tests also pass locally.