# Understanding asyncio.subprocess.Process garbage collection — discuss.python.org

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | async-pty-process-testing |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Understanding asyncio.subprocess.Process garbage collection |
| Source URL | https://discuss.python.org/t/understanding-asyncio-subprocess-process-garbage-collection/105973 |
| Source publisher / author | Python Discourse community |
| Source type | forum thread |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-09 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | Medium |
| Status | Active |

## Summary

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.

## Key findings

- `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.
- 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.

## Pros / strengths

- 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.

## Cons / limitations / caveats

- 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.

## Relevance to tai

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.

## Changelog

| Date | Change |
|---|---|
| 2026-10-09 | Initial dossier created. |