48 lines
2.7 KiB
Markdown
48 lines
2.7 KiB
Markdown
 
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# Understanding asyncio.subprocess.Process garbage collection — discuss.python.org
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## Metadata
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| Field | Value |
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| Institute | Molodetz Institute — Research & Retrieval Division |
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| Subject | async-pty-process-testing |
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| Document type | Source Dossier |
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| Tier | 1 |
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| Source title | Understanding asyncio.subprocess.Process garbage collection |
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| Source URL | https://discuss.python.org/t/understanding-asyncio-subprocess-process-garbage-collection/105973 |
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| Source publisher / author | Python Discourse community |
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| Source type | forum thread |
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| Source date (published) | Unknown — not stated |
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| Date researched | 2026-10-09 |
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| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
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| Research method | web search synthesis |
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| Confidence | Medium |
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| Status | Active |
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## Summary
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A Discourse thread examining what happens to a still-running `asyncio.subprocess.Process` object when it's garbage collected. The stdlib documentation states the child gets killed as a safety net in that case — but a forum participant reported observing output continuing to appear after the "owning" script had already finished, suggesting this safety net is not reliably instantaneous or complete in practice.
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## Key findings
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- `asyncio.subprocess.Process`'s own documentation states that if the process object is garbage collected while the process is still running, the child process will be killed — a documented safety net, not an accident of implementation.
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- Despite that documented behavior, a thread participant reported a case where a subprocess's "Step" output kept appearing after the main script had already exited, implying the GC-triggered kill either didn't fire promptly or didn't fully suppress output in that instance.
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## Pros / strengths
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- A concrete counter-example to "just let Python's GC clean it up" as a cleanup strategy — directly useful as evidence against relying on implicit cleanup.
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## Cons / limitations / caveats
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- A single forum report of unexpected behavior, not a reproduced, isolated bug report with a minimal test case — the exact conditions that produced it were not independently verified in this research pass.
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## Relevance to tai
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Reinforced the decision that `tai_test.py.TestCase.execute()` must explicitly call `await proc.terminate()` for every spawned process in its `finally` block, regardless of whether the test passed, failed, or was cancelled — never relying on Python's garbage collector or process-object destruction to clean anything up, consistent with the companion finding in `bpo-wait-for-timeout-leaves-process-alive.md` that a bare timeout alone doesn't guarantee cleanup either.
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## Changelog
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| Date | Change |
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| 2026-10-09 | Initial dossier created. |
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