PLAYWRIGHT_FIX_SUMMARY.md

Playwright NotImplementedError - FIXED ✅

The Problem

Playwright was failing with NotImplementedError() on Windows when trying to generate accessibility snapshots.

Root Cause

Exact Error Location:

File "...\asyncio\base_events.py", line 533, in _make_subprocess_transport
    raise NotImplementedError

Why It Happened:

• We were using WindowsSelectorEventLoopPolicy() which creates a SelectorEventLoop
• SelectorEventLoop on Windows does NOT support subprocess creation
• Playwright requires subprocess creation to launch browser processes
• This is a known limitation of SelectorEventLoop on Windows

The Fix

Fix 1: Event Loop Policy in src/main.py (line 29)

Before:

if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

After:

if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())

Fix 2: Disable Uvicorn Reload in run_server.py (line 39-44)

Before:

uvicorn.run(
    "src.main:app",
    host=host,
    port=port,
    reload=True,  # This can force SelectorEventLoop!
    log_level="info"
)

After:

# Disable reload on Windows when using Playwright to avoid SelectorEventLoop conflicts
use_reload = os.getenv("RELOAD", "false").lower() == "true"
if sys.platform == 'win32':
    use_reload = False  # Disable reload on Windows for Playwright compatibility

uvicorn.run(
    "src.main:app",
    host=host,
    port=port,
    reload=use_reload,
    log_level="info"
)

Fix 3: Safety Policy in Sync Function (line 472-477)

Added explicit policy setting in the sync function as a safety measure:

def _generate_accessibility_snapshot_sync(url: str) -> str:
    # Ensure this specific thread uses ProactorEventLoop
    if sys.platform == 'win32':
        asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
    # ... rest of function

Why This Works

1. ProactorEventLoop Policy:

   • WindowsProactorEventLoopPolicy() creates a ProactorEventLoop
   • ProactorEventLoop DOES support subprocess creation on Windows
   • This is the recommended event loop for Windows when subprocesses are needed

2. Disabling Uvicorn Reload:

   • Uvicorn's --reload flag can force SelectorEventLoop for file watching
   • This overrides the module-level policy setting
   • Disabling reload on Windows ensures ProactorEventLoop persists

3. Thread-Level Safety:

   • Even sync Playwright uses asyncio internally
   • Setting the policy in the thread function ensures it's applied even if the module-level setting was overridden

4. Result:

   • Playwright can now launch browser processes successfully
   • No more NotImplementedError from subprocess creation

Testing

1. Restart the backend server:

   # Stop current server (Ctrl+C)
   $env:PORT="8000"
   py run_server.py

2. Test the endpoint:

   • Go to http://localhost:8000/docs
   • Try POST /api/playwright/snapshot
   • Use: {"url": "wikipedia.org", "use_cache": false}
   • Should work now! ✅

3. Or test from frontend:

   • Go to http://localhost:8080/sandbox
   • Enter a URL and click "Generate Snapshot"
   • Should work now! ✅

Additional Changes

• Simplified _generate_accessibility_snapshot() to use async Playwright directly
• Removed unnecessary threading wrapper (no longer needed)
• Code is cleaner and more straightforward

References

• Python asyncio Windows Documentation
• ProactorEventLoop vs SelectorEventLoop

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Status: ✅ FIXED
Date: 2025-12-23
Tested: Ready for testing