# Non blocking file reader for python asyncio · GitHub Gist

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | async-pty-process-testing |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Non blocking file reader for python asyncio |
| Source URL | https://gist.github.com/nilp0inter/5f400b6afb6df7c0b553 |
| Source publisher / author | GitHub user nilp0inter |
| Source type | repository (gist) |
| 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 | High |
| Status | Active |

## Summary

A GitHub gist demonstrating the correct way to read stdin (or any fd) without blocking the asyncio event loop: use `loop.add_reader(fd, callback)` where the callback does a raw `os.read(fd, n)`, never a call through Python's buffered `sys.stdin` object, which blocks regardless of `add_reader`.

## Key findings

- Calling `sys.stdin.read()` inside an `add_reader` callback still blocks, because Python's buffered stdin object waits for more data internally — `add_reader` only tells the event loop the fd is readable, it does not change how a buffered read call behaves.
- The correct pattern is `os.read(fileno, n)` on the raw file descriptor inside the callback.
- On EOF (an empty read), the callback should call `loop.remove_reader(fileno)` to stop watching the fd.

## Pros / strengths

- Identifies a genuine, easy-to-make mistake (buffered read inside a non-blocking callback) with a precise fix, backed by a reported working example.
- The pattern is minimal and requires no third-party dependency — pure stdlib (`os`, `asyncio`).

## Cons / limitations / caveats

- A gist, not an authoritative reference — no discussion of edge cases like partial reads mid-line or how this interacts with terminal raw/cooked mode.
- Only covers the read side; doesn't address writing back to the child or coordinating with a separate output stream.

## Relevance to tai

This is the exact pattern `tai_test.py`'s `StdinBridge._on_readable()` implements: `os.read(self._fd, 4096)` inside an `add_reader` callback, buffering until a newline, and calling `self.stop()` (which internally calls `loop.remove_reader`) on an empty read. Using `sys.stdin.read()` instead would have silently reintroduced blocking behavior that defeats the entire point of the non-blocking design.

## Changelog

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