oh-my-pi/docs/python-repl.md

Eval Tool Python Backend

This document describes the Python execution stack in packages/coding-agent. It covers tool behavior, runner lifecycle, environment handling, execution semantics, output rendering, supported magics, and operational failure modes.

Scope and Key Files

• Tool surface: src/tools/eval.ts
• Session/per-call kernel orchestration: src/eval/py/executor.ts
• Subprocess kernel client: src/eval/py/kernel.ts
• Python wrapper / NDJSON server: src/eval/py/runner.py
• Prelude helpers loaded into every kernel: src/eval/py/prelude.py
• MIME bundle renderer (text + structured outputs): src/eval/py/display.ts
• Interactive-mode renderer for user-triggered Python runs: src/modes/components/eval-execution.ts
• Runtime/env filtering and Python resolution: src/eval/py/runtime.ts

What eval's Python backend is

The eval tool executes one or more Python cells inside a retained python subprocess that speaks NDJSON over stdin/stdout. No Jupyter gateway and no extra pip dependencies are required — a vanilla Python 3.8+ interpreter is enough. Rich display() output (PIL, pandas, plotly, matplotlib figures) keeps working because the wrapper implements MIME-bundle dispatch.

Tool params:

{
  cells: Array<{
    language: "py" | "js";
    code: string;
    title?: string;
    timeout?: number; // seconds, clamped to 1..600, default 30
    reset?: boolean; // reset this cell's selected runtime before execution
  }>;
}

The tool is concurrency = "exclusive" for a session, so calls do not overlap.

Kernel lifecycle

Each Python kernel is a single subprocess: <resolved-python> -u <runner.py>. The runner is bundled with the host binary (Bun text import), written to an omp-python-runner cache under the OS temp directory once per script hash, and reused by subsequent spawns.

Kernel startup sequence:

1. Availability check (checkPythonKernelAvailability) — verifies that a Python interpreter resolves and runs.
2. Spawn python -u runner.py with filtered env and cwd.
3. Send an init request that runs os.chdir(cwd), injects env entries, and adds cwd to sys.path.
4. Execute PYTHON_PRELUDE (idempotent — only initializes once per process).

Kernel shutdown:

• Send {"type": "exit"} over stdin.
• Wait for process exit with SHUTDOWN_GRACE_MS budget.
• Escalate to SIGTERM and finally SIGKILL if the process does not exit in time.

Wire protocol (NDJSON, host ↔ runner)

One JSON object per line, UTF-8, \n terminated.

Host → runner:

{"id": "<reqId>", "code": "<source>", "silent": false, "storeHistory": true}
{"type": "exit"}

Runner → host:

{"type": "started",  "id": "<reqId>"}
{"type": "stdout",   "id": "<reqId>", "data": "..."}
{"type": "stderr",   "id": "<reqId>", "data": "..."}
{"type": "display",  "id": "<reqId>", "bundle": {<mime>: <value>}}
{"type": "result",   "id": "<reqId>", "bundle": {<mime>: <value>}}
{"type": "error",    "id": "<reqId>", "ename": "...", "evalue": "...", "traceback": ["..."]}
{"type": "done",     "id": "<reqId>", "status": "ok"|"error", "executionCount": N, "cancelled": false}

Status events the prelude emits (e.g. _emit_status("find", count=…)) ship inside display bundles under application/x-omp-status so the existing TUI status renderer keeps working.

Magics

The runner's source transformer rewrites IPython-style magics to plain Python calls before parsing. Supported set:

Magic	Effect
%pip <args>	python -m pip <args> with live streaming output. Newly installed packages are evicted from sys.modules so the next import picks up the fresh install.
%cd <path>	os.chdir(path) (with ~ expansion); emits status event.
%pwd	Returns os.getcwd().
%ls [path]	Returns sorted(os.listdir(path)).
%env [KEY[=VAL]]	List, read, or set env vars (matches prelude env() semantics).
%set_env KEY VALUE	Set os.environ[KEY].
%time <expr> / %timeit <expr>	Time the expression; emits status event with elapsed ms.
%who / %whos	List user-namespace names.
%reset	Clear user globals and re-inject prelude.
%load <path>	Read a file into a fresh cell and execute.
%run <path>	runpy.run_path and merge globals back.
%%bash / %%sh	Run the cell body via bash/sh.
%%capture [name]	Run body with stdout/stderr captured into name.
%%timeit	Time the cell body.
%%writefile <path>	Write body to file.
!cmd / var = !cmd	Run command via subprocess shell; returns an SList-style result with .n / .s helpers.
var = %name args	Assignment forms work for line magics and !cmd.

Unknown magic names raise NameError: UsageError: ... inside the cell.

Session persistence semantics

python.kernelMode controls retained kernel reuse:

• session (default)
  • Reuses kernel sessions keyed by namespaced eval session id plus cwd.
  • Multiple owners can share the same retained kernel for that key.
  • Calls through the tool are exclusive, so tool invocations do not overlap.
  • A dead retained subprocess is replaced before execution.
  • If the subprocess dies during execution, it is replaced and the cell is retried once.
• per-call
  • Spawns a fresh subprocess for each request.
  • Shuts the subprocess down after the request.
  • No cross-call state persistence.

Multi-cell behavior in a single tool call

Python cells run sequentially in the same selected Python kernel instance for that tool call.

If an intermediate cell fails:

• Earlier cell state remains in memory.
• Tool returns a targeted error indicating which cell failed.
• Later cells are not executed.

reset=true is per cell and resets that language runtime before the cell executes.

Environment filtering and runtime resolution

Environment is filtered before launching the runner:

• Allowlist includes core vars like PATH, HOME, locale vars, VIRTUAL_ENV, PYTHONPATH, etc.
• Allow-prefixes: LC_, XDG_, PI_
• Denylist strips common API keys (OpenAI/Anthropic/Gemini/etc.)

Runtime selection order:

1. Active/located venv (VIRTUAL_ENV, then <cwd>/.venv, <cwd>/venv)
2. Managed venv at ~/.omp/python-env
3. python or python3 on PATH

When a venv is selected, its bin/Scripts path is prepended to PATH.

The runner additionally receives PYTHONUNBUFFERED=1 and PYTHONIOENCODING=utf-8 so streamed output reaches the host promptly.

Tool availability and mode selection

eval.py / eval.js (both default true) plus optional boolean env flags PI_PY / PI_JS control eval backend exposure:

• Python backend only (eval.py=true, eval.js=false, or PI_PY=1 PI_JS=0)
• JavaScript backend only (eval.py=false, eval.js=true, or PI_PY=0 PI_JS=1)
• both backends (eval.py=true, eval.js=true, or PI_PY=1 PI_JS=1)

PI_PY and PI_JS use normal boolean flag parsing. If either env var is set, the env pair overrides the per-key settings; an unset member of the pair defaults to enabled.

If Python preflight fails and eval.js is enabled, eval remains available for js cells; py cells fail with a Python-backend availability error.

Execution flow and cancellation/timeout

Cell timeout

Each eval cell timeout is in seconds, defaults to 30, and is clamped to 1..600. The tool combines caller abort signal, session abort signal, and the current cell timeout with AbortSignal.any(...).

Kernel execution cancellation

On abort/timeout:

• The host sends kill("SIGINT") to the runner subprocess.
• The runner's exec-time signal handler raises KeyboardInterrupt inside the user code.
• Result includes cancelled=true; timeout path annotates output as Command timed out after <n> seconds.
• Between requests the runner installs SIG_IGN for SIGINT so a stray cancel does not tear down the kernel.

If a second cancel is required (runner stuck in C code), the host escalates to SIGTERM and the session restarts on the next call.

stdin behavior

Interactive stdin is not supported. The runner does not forward input() prompts; user code that calls input() blocks until cancellation.

Output capture and rendering

Captured output classes

From runner frames:

• stdout / stderr → plain text chunks
• display / result → rich display handling (MIME bundle)
• error → traceback text
• application/x-omp-status MIME inside display → structured status events

Display MIME precedence:

1. text/markdown
2. text/plain
3. text/html (converted to basic markdown)

Additionally captured as structured outputs:

• application/json → JSON tree data
• image/png / image/jpeg → image payloads
• application/x-omp-status → status events

Matplotlib

The runner sets MPLBACKEND=Agg as an environ default so figures render off-screen. After every cell, pyplot.get_fignums() is iterated; each figure is saved to PNG, emitted as an image/png display, and closed.

Storage and truncation

Output is streamed through OutputSink and may be persisted to artifact storage. Tool results can include truncation metadata and artifact://<id> for full output recovery.

Renderer behavior

• Tool renderer (eval.ts):
  • shows code-cell blocks with per-cell status
  • collapsed preview defaults to 10 lines
  • supports expanded mode for all output retained in the tool result
• Interactive renderer (eval-execution.ts):
  • used for user-triggered Python execution in TUI
  • collapsed preview defaults to 20 lines
  • clamps very long individual lines to 4000 chars for display safety
  • shows cancellation/error/truncation notices

Operational troubleshooting

• Python backend not available — Check eval.py, PI_PY, and that python/python3 is on PATH. If preflight fails and eval.js is enabled, use a js cell.
• No Python on PATH — Install a system Python 3.8+ or place a venv at ~/.omp/python-env. omp setup python --check reports the resolved interpreter.
• Execution hangs then times out — Increase tool timeout (max 600s) if workload is legitimate. For stuck native code, cancellation triggers SIGINT first then escalates; the session restarts on the next request.
• stdin/input prompts in Python code — input() is not supported; pass data programmatically.
• Working directory errors — Tool validates cwd exists and is a directory before execution.

Relevant environment variables

• PI_PY / PI_JS — eval backend exposure overrides
• PI_PYTHON_SKIP_CHECK=1 — bypass Python preflight/warm checks
• PI_PYTHON_INTEGRATION=1 — enable gated integration tests that spawn a real Python
• PI_PYTHON_IPC_TRACE=1 — log NDJSON frames exchanged with the runner subprocess