Runtimeerror: this event loop is already running

The runtimeerror: this event loop is already running error is a common error that developers may encounter when working with asyncio in Python.

This error message typically occurs when you are attempting to run an event loop in Python more than once.

In this article, we will provide complete details on how to resolve this error and provide helpful tips to avoid encountering this error in the future.

Reproduce the Error

import asyncio

async def foo():
    print("foo")

async def main():
    asyncio.get_running_loop().run_until_complete(foo())
    asyncio.run(foo())

asyncio.run(main())

Output:

RuntimeError: This event loop is already running

This error message shows that you are attempting to start a new event loop while another event loop is already running, which is not allowed.

Why does runtimeerror this event loop is already running error occur?

This runtimeerror this event loop is already running error typically occurs because we are attempting to start a new event loop while an existing one is already running.

In every program, there is only one event loop at a time, so attempting to create a new one while an existing one is running will cause an error.

How to Fix the Error?

There are various methods to fix the this event loop is already running error. Here are some solutions that will be able to help you resolve this error:

Solution 1: Using asyncio.get_event_loop() to solve the error

One of the solutions to solve this error is to use the asyncio.get_event_loop() function. This function restores the current event loop if one is already running, or generates a new one if none exists.

Here is an example of how to use this method:

import asyncio

# get the event loop
loop = asyncio.get_event_loop()

# use the event loop to run your coroutine(s)
async def my_eventLoop():
    # your coroutine code here
    await asyncio.sleep(1)
    print("My Loop is finished executing.")

loop.run_until_complete(my_eventLoop())

Output:

My Loop is finished executing.

Solution 2: Creating a new event loop to solve the error

The another solution to solve this error is to create a new event loop if one does not exist.

This is an example of how to create a new event loop:

import asyncio

async def looping_one():
    # looping one code here
    await asyncio.sleep(1)
    print("The looping one finished executing.")

async def looping_two():
    
    await asyncio.sleep(2)
    print("The looping two finished executing.")

async def main():
    # create a new event loop
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)

    # run multiple loop concurrently using `gather`
    await asyncio.gather(
        looping_one(),
        looping_two()
    )

if __name__ == '__main__':
    # run the main looping using the `run` method
    asyncio.run(main())

Output:

The looping one finished executing.
The looping two finished executing.

Solution 3: Closing the event loop to solve the error

If none of the above solutions is working, then you can try closing the event loop manually.

Here is an example code on how to do this:

import asyncio

async def my_coroutine():
    # Do some asynchronous work here
    pass

async def main():
    # Create an event loop
    loop = asyncio.get_event_loop()

    # Run the coroutine
    await my_coroutine()

    # Close the event loop
    loop.close()

if __name__ == '__main__':
    # Run the main coroutine using the asyncio.run() method
    asyncio.run(main())

Tips to Avoid this Error

The following are helpful tips to prevent encountering this error in the future:

• Avoid running the event loop multiple times.
• Make sure that the event loop is properly closed when it is no longer needed.
• Use the asyncio.get_running_loop() method instead of asyncio.get_event_loop() to retrieve the current event loop.

Additional Resources

Here are the following articles that will be able to help you to understand more about runtimeerror:

• Runtimeerror: dictionary changed size during iteration

• Cannot add middleware after an application has started

• Runtimeerror: expected scalar type float but found double

Conclusion

In conclusion, the “runtime error: this event loop is already running” issue can be frustrating for programmers, but with the appropriate knowledge, it can be resolved quickly and easily.

By following the solutions outlined in this article, you can fix this issue and avoid encountering it in the future.

We hope that this article has been informative and helpful in resolving the “runtimeerror: this event loop is already running” error.

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