asyncio.get_event_loop() DeprecationWarning

Problem Statement

When working with Python's asyncio library, you may encounter the warning:

DeprecationWarning: There is no current event loop
loop = asyncio.get_event_loop()

This warning appears when your code tries to get the current event loop using asyncio.get_event_loop() in a context where no event loop has been set for the current thread. Starting from Python 3.11, this situation raises a RuntimeError instead of just a warning, breaking your code.

The issue typically occurs in main entry points or when working with multithreaded applications where the event loop hasn't been properly initialized.

Solution

Modern Python Best Practice (Python 3.7+)

The recommended approach is to use asyncio.run() for your main entry point:

async def main():
    # Your async code here
    pass

if __name__ == '__main__':
    asyncio.run(main())

When You Need Explicit Loop Access

If you need explicit access to the event loop (for example, when using libraries that require loop passing), properly create and set the loop:

if __name__ == '__main__':
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    try:
        loop.run_until_complete(amain(loop=loop))
    except KeyboardInterrupt:
        pass

Detailed Explanation

Why the Warning Occurs

Prior to Python 3.10, asyncio.get_event_loop() would create a new event loop if one didn't exist in the current thread. This behavior was deprecated and removed because:

1. It could lead to unexpected behavior when multiple loops were created
2. It made thread safety issues more likely
3. The implicit creation made debugging more difficult

Migration Strategies

Before (Problematic)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()  # Warning/Error
    loop.create_task(amain(loop=loop))
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass

After (Fixed)

if __name__ == '__main__':
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    try:
        loop.run_until_complete(amain(loop=loop))
    except KeyboardInterrupt:
        pass

Best Practices

1. Use asyncio.run() when possible - It handles loop creation and cleanup automatically
2. Avoid passing loops explicitly - Use asyncio.get_running_loop() inside async functions
3. Modify existing functions to obtain the running loop internally:

async def amain():
    loop = asyncio.get_running_loop()  # Get current running loop
    # Rest of your implementation

Important Considerations

WARNING

loop.run_until_complete() doesn't clean up asynchronous generators like asyncio.run() does. If you're using async generators, you may need to handle cleanup manually.

INFO

For library code, avoid creating event loops entirely. Instead, assume the caller will provide a running event loop and use asyncio.get_running_loop() to access it.

Complete Working Example

Here's a complete example showing the proper way to structure an async application:

import asyncio
import aiosmtpd

async def amain():
    # Get the currently running loop
    loop = asyncio.get_running_loop()
    
    # Your SMTP server setup here
    # controller = await loop.create_server(...)
    
    # Keep running until interrupted
    try:
        await asyncio.Future()  # Run forever
    except KeyboardInterrupt:
        print("Server stopped")

if __name__ == '__main__':
    asyncio.run(amain())

This approach works with modern Python versions (3.7+) and avoids the deprecation warning while following current best practices.