# 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:**
```python
if sys.platform == 'win32':
asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
```
**After:**
```python
if sys.platform == 'win32':
asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
```
### Fix 2: Disable Uvicorn Reload in `run_server.py` (line 39-44)
**Before:**
```python
uvicorn.run(
"src.main:app",
host=host,
port=port,
reload=True, # This can force SelectorEventLoop!
log_level="info"
)
```
**After:**
```python
# 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:
```python
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:**
```powershell
# 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](https://docs.python.org/3/library/asyncio-platforms.html#windows)
- [ProactorEventLoop vs SelectorEventLoop](https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.ProactorEventLoop)
---
**Status:** ✅ FIXED
**Date:** 2025-12-23
**Tested:** Ready for testing
