Windows: PyodideSandbox subprocess support broken due to event loop policy incompatibility

[gpavlov2016]

Problem

When using PyodideSandbox from langchain_sandbox within LangGraph dev environment on Windows, subprocess operations fail with NotImplementedError. This prevents testing and using Deno-based sandbox functionality on Windows.

Error Details

Traceback (most recent call last):
  File "path/to/tools.py", line 296, in python_repl_tool_sandbox
    result = await sandbox.execute(wrapper_code)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path/to/langchain_sandbox/pyodide.py", line 298, in execute
    process = await asyncio.create_subprocess_exec(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path/to/asyncio/subprocess.py", line 223, in create_subprocess_exec
    transport, protocol = await loop.subprocess_exec(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path/to/asyncio/base_events.py", line 1708, in subprocess_exec
    transport = await self._make_subprocess_transport(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "path/to/asyncio/base_events.py", line 503, in _make_subprocess_transport
    raise NotImplementedError
NotImplementedError

Root Cause

LangGraph uses _WindowsSelectorEventLoop on Windows, which doesn't support subprocess operations. PyodideSandbox requires subprocess support to spawn Deno processes for code execution.

Reproduction Steps

1. Set up LangGraph dev environment on Windows
2. Install langchain_sandbox
3. Create a tool using PyodideSandbox:

from langchain_sandbox import PyodideSandbox

sandbox = PyodideSandbox(allow_net=True)

async def test_sandbox():
    result = await sandbox.execute("print('Hello, world!')", timeout_seconds=10)
    return result

4. Call the tool within LangGraph
5. Observe NotImplementedError

Expected Behavior

• PyodideSandbox should work on Windows within LangGraph dev environment
• Subprocess operations should be supported
• Deno-based code execution should function normally

Actual Behavior

• NotImplementedError when trying to create subprocesses
• Sandbox functionality completely broken on Windows
• No workaround available without significant code changes

Environment

• OS: Windows 10/11
• Python: 3.11+
• LangGraph: Latest dev version
• langchain_sandbox: Latest version

Workarounds Attempted

1. Setting event loop policy before imports: asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())

Impact

• Prevents testing Deno functionality on Windows
• Blocks Windows developers from using sandbox features
• Forces workarounds that add complexity and potential issues

Proposed Solutions

1. Option A: Allow configurable event loop policy in LangGraph
2. Option B: Use ProactorEventLoop on Windows by default
3. Option C: Add subprocess support to _WindowsSelectorEventLoop

Related Issues

• This affects any tool that requires subprocess support on Windows
• Similar issues may exist with other async libraries that depend on subprocesses

Additional Context

• Works fine on Linux/macOS
• Works fine when running outside LangGraph environment langgraph dev
• Issue is specific to Windows + LangGraph combination

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Component: Core/Event Loop