# Subprocesses — Python 3 documentation (asyncio.subprocess)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | async-pty-process-testing |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Subprocesses — Python 3.14 documentation |
| Source URL | https://docs.python.org/3/library/asyncio-subprocess.html |
| Source publisher / author | Python Software Foundation |
| Source type | standard (official stdlib documentation) |
| Source date (published) | Unknown — not stated (continuously updated) |
| Date researched | 2026-10-09 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | High |
| Status | Active |

## Summary

The canonical stdlib reference for `asyncio.subprocess`, confirming the exact semantics of `terminate()`/`kill()`, how `Process.stdout`/`Process.stderr` expose `StreamReader` instances when `PIPE` is used, and the deadlock risk of reading those streams manually instead of via `communicate()`.

## Key findings

- On POSIX, `Process.terminate()` sends SIGTERM to the child; `Process.kill()` sends SIGKILL. On Windows, `terminate()` calls the Win32 `TerminateProcess` API; there is no separate POSIX-signal distinction.
- When `stdout=PIPE` or `stderr=PIPE` is passed, `Process.stdout`/`Process.stderr` are `asyncio.StreamReader` instances — the same abstraction `tai_test.py` wraps a pty master fd into via `loop.connect_read_pipe`, for a uniform interface across both transports.
- The docs warn that reading those streams manually (rather than via `communicate()`) can deadlock, because the child can block on a full pipe buffer if its output isn't drained quickly enough while it's also waiting on something else.

## Pros / strengths

- Authoritative, primary-source documentation — the single most load-bearing source in this entire subject for API semantics.

## Cons / limitations / caveats

- Documents stdlib behavior as of whatever version was fetched; does not itself discuss process-group-wide termination (`killpg`) or the controlling-terminal/pty nuances this subject's other sources cover — those required separate sources.

## Relevance to tai

The definitive basis for `tai_test.py`'s choice of `terminate()` (SIGTERM) before escalating to `kill()` (SIGKILL) in `ManagedProcess.terminate()`'s escalation loop, and for exposing `Process.stdout`/`Process.stderr` as the pipe-mode reader rather than hand-rolling a reader over raw fds in that transport. The deadlock warning is why `ManagedProcess.drain_until_eof()` continuously reads in a loop until EOF rather than attempting a single large read.

## Changelog

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