Playwright Exception on Windows: No Description in FastAPI App

Playwright is a popular automation tool for web testing and scraping, offering powerful features for interacting with websites. It works seamlessly with FastAPI when used correctly, but some developers on Windows have run into a cryptic issue—the No Description error message.

At first glance, the error provides little information, leaving users frustrated. But there’s a clear technical reason behind it, and fortunately, a simple fix. Let’s break it down and resolve this issue without headaches.

Understanding the Issue: The Playwright Exception

Developers encountering this issue typically use Playwright inside a FastAPI route. When a request triggers Playwright, instead of running smoothly, it fails with an exception.

Visualizing the Error

Notice the key problem: there’s no helpful description—just a failure.

Where This Happens: FastAPI and Windows

The error is commonly reported when using Playwright inside an asynchronous FastAPI endpoint running on Windows. It doesn’t always happen in direct script execution, making it more confusing. Many of these cases involve `async_playwright`, meaning FastAPI is running Playwright in an async event loop, which behaves differently on Windows than on Linux.

Reproducing the Issue

Here’s a small FastAPI app that triggers the error.

from fastapi import FastAPI
import asyncio
from playwright.async_api import async_playwright

app = FastAPI()

async def run_playwright():
    async with async_playwright() as p:
        browser = await p.chromium.launch()
        page = await browser.new_page()
        await page.goto("https://www.example.com")
        await browser.close()

@app.get("/")
async def root():
    await run_playwright()
    return {"message": "Playwright executed!"}

Run this app with Uvicorn:

uvicorn main:app --reload

Then visit `http://127.0.0.1:8000/` in a browser or use curl:

curl http://127.0.0.1:8000

If you’re on Windows, you might see the Playwright error instead of the expected success message.

The Root Cause: Asyncio’s Event Loop on Windows

The core issue is Windows’ default async event loop, which behaves differently from other platforms.

The Problem: SelectorEventLoop vs. ProactorEventLoop

Windows supports two types of event loops in Python:

• SelectorEventLoop: Uses `select()` internally, which works well for basic async tasks but has known issues with subprocesses.
• ProactorEventLoop: Uses Windows I/O Completion Ports (IOCP), making it better suited for handling Playwright and other async-heavy libraries.

On Windows, FastAPI (via Uvicorn) may default to SelectorEventLoop, which is incompatible with Playwright.

How Playwright Depends on ProactorEventLoop

Playwright relies on subprocesses to launch browsers like Chrome and Firefox. When using SelectorEventLoop, these subprocesses don’t start correctly, triggering cryptic errors like No Description.

Checking the Current Event Loop

Run this snippet to see which event loop is active:

import asyncio
import sys

if sys.platform == "win32":
    loop = asyncio.get_event_loop()
    print(type(loop))

If it prints ``, you’ve found the culprit.

Resolving the Issue: Using the Right Event Loop

Solution 1: Explicitly Set ProactorEventLoop

Before starting FastAPI, set the correct event loop:

import asyncio
import sys

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

# Now run FastAPI

This ensures the ProactorEventLoop is used instead of the default one.

Solution 2: Use `nest_asyncio` (Alternative Fix)

If changing the loop globally isn’t an option (e.g., in Jupyter notebooks or embedded environments), use `nest_asyncio`:

import nest_asyncio
nest_asyncio.apply()

This patches the event loop, allowing nested coroutines to work correctly.

Comparing Solutions

For FastAPI, ProactorEventLoop is the better choice because it aligns with Playwright’s backend execution.

Best Practices and Troubleshooting

Verify the Fix

After applying the fix, rerun the event loop check script to confirm it’s using `ProactorEventLoop`.

Keep Dependencies Up to Date

Ensure Playwright, FastAPI, and Uvicorn are up to date:

pip install --upgrade playwright fastapi uvicorn

Check for Other Potential Causes

Other factors might cause similar issues:

• Incorrect Playwright installation: Run `playwright install` to ensure browsers are installed.
• Firewall Restrictions: Some antivirus and firewalls block Playwright’s subprocess handling.
• Network Issues: If Playwright needs an internet connection, ensure proper permissions are set.

Advanced Considerations

Using Docker or WSL

An alternative to managing Windows-specific quirks is running Playwright inside a Linux container or WSL.

– Docker: Runs Playwright in an isolated Linux environment.
– WSL: Allows running Python in a Linux-like subsystem.

Both approaches bypass Windows’ asyncio quirks but add complexity.

Closing Remarks

The No Description error in Playwright on Windows isn’t random—it’s caused by an incompatible async event loop. By configuring Windows to use ProactorEventLoop, you ensure Playwright executes correctly inside FastAPI. If you’ve run into similar async issues, let us know what worked for you!