DeprecationWarning: there is no current event loop

The DeprecationWarning: there is no current event loop is a common that arises while working with loops in Python. Though Python is a user-friendly programming language, it introduces challenges and complexities, such as handling errors and managing event loops.

An event loop is a core component of asynchronous programming in Python. It is responsible for executing and scheduling coroutines. The coroutines are special functions that can pause and resume their execution. An event loop also handles other events, such as I/O operations, timers, signals, and callbacks.

This guide will give you a walkthrough about this error and will help you resolve this error and provide some FAQs.

What is the “DeprecationWarning: there is no current event loop” error?

The DeprecationWarning: there is no current event loop error is caused when you try to access an event loop that does not exist for the current environment. Another reason why the “DeprecationWarning: there is no current event loop” error appears when the code has not been set for the current thread.

This warning was introduced in Python 3.10 to discourage using the asyncio.get_event_loop() function. It returns the current event loop for the current thread or creates a new one if none exists. This function can cause confusion and unexpected behavior, especially when working with multiple threads or processes.

For example, if a new event loop is created implicitly by asyncio.get_event_loop() may not be properly configured or closed, leading to resource leaks or errors.

What causes the “DeprecationWarning: there is no current event loop” error?

The “DeprecationWarning: there is no current event loop” error is caused by calling the asyncio.get_event_loop() function without ensuring that an event loop exists and is set for the current thread. This function is deprecated in Python 3.10 and will raise an error in Python 3.11, as it can cause confusion and unexpected behavior. Various scenarios can trigger this error, such as:

Calling asyncio.get_event_loop() before calling asyncio.run()

The recommended way to run an asynchronous program and create an event loop in Python is to use the asyncio.run() function. This function will handle the creation, configuration, and closing of the event loop and raise an error if called more than once; however, if you call asyncio.get_event_loop() before calling asyncio.run(), you may implicitly create a new event loop that is not properly configured or closed, leading to resource leaks or errors.

Calling asyncio.get_event_loop() in a different thread than the one where the event loop was created or set

Each thread in Python has its event loop. If you create or set an event loop in one thread, you cannot access it from another thread using asyncio.get_event_loop().

Instead, you need to pass the event loop explicitly as an argument to functions that need it or use the asyncio.run_coroutine_threadsafe() function to submit a coroutine to an existing event loop in another thread.

Calling asyncio.get_event_loop() after the event loop has been closed or destroyed

An event loop can be closed or destroyed by calling the loop.close() method or by using the asyncio.run() function, which will close the event loop automatically after running the main coroutine.

If you try to access the event loop after it has been closed or destroyed, you will get the “DeprecationWarning: there is no current event loop” error. You must manually create and set a new event loop using the asyncio.new_event_loop() and asyncio.set_event_loop() functions if you want to use an event loop again.

Calling asyncio.get_event_loop() in a function that is not marked as async

A function marked async is a coroutine, meaning the event loop can run it. A function not marked async is a regular function, which means that the event loop cannot run it. Suppose you call asyncio.get_event_loop() in a regular function.

In that case, you may get the “DeprecationWarning: there is no current event loop” error because there is no running event loop in the current thread. You need to mark the function as async or use the loop.run_in_executor() method to run it in a thread or process pool executor.

Here is an example of code that can cause the “DeprecationWarning: there is no current event loop” error:

import asyncio

def sync_func():
    # This function is not a coroutine and cannot be run by the event loop
    loop = asyncio.get_event_loop() # This may create a new event loop implicitly
    loop.run_until_complete(async_func()) # This may cause an error or a warning

async def async_func():
    # This function is a coroutine and can be run by the event loop
    print("Hello from async_func")

sync_func()

How do you resolve the “DeprecationWarning: there is no current event loop” error?

The “DeprecationWarning: there is no current event loop” error can be resolved by following the best practices for using event loops in Python. Some of the general guidelines are:

Use asyncio.run() to run the main coroutine of the program and create an event loop

This function will handle the creation, configuration, and closing of the event loop and raise an error if called more than once. For example:

import asyncio

async def main():
    # This is the main coroutine of the program
    print("Hello from main")

asyncio.run(main()) # This runs the main coroutine and creates an event loop

Avoid using asyncio.get_event_loop() to access the current event loop

Another solution to resolve the DeprecationWarning: there is no current event loop error is to use asyncio.get_running_loop() to get the running event loop in a coroutine or explicitly pass it as an argument to functions that need it. For example:

import asyncio

async def coro1(loop):
    # This function is a coroutine and takes the event loop as an argument
    print("Hello from coro1")

async def coro2():
    # This function is a coroutine and gets the running event loop
    loop = asyncio.get_running_loop()
    print("Hello from coro2")

async def main():
    # This is the main coroutine of the program
    loop = asyncio.get_running_loop() # This gets the running event loop
    await asyncio.gather(coro1(loop), coro2()) # This runs the coroutines concurrently and passes the event loop to coro1

asyncio.run(main()) # This runs the main coroutine and creates an event loop

If you need to create and set a new event loop manually, use asyncio.new_event_loop() and asyncio.set_event_loop()

Close the event loop properly when it is no longer needed. For example:

import asyncio

async def main():
    # This is the main coroutine of the program
    print("Hello from main")

# Create a new event loop
loop = asyncio.new_event_loop()
# Set the event loop for the current thread
asyncio.set_event_loop(loop)
# Run the main coroutine
loop.run_until_complete(main())
# Close the event loop
loop.close()

If you need to run a coroutine from a synchronous function, use asyncio.run_coroutine_threadsafe()

This function will submit the coroutine to an existing event loop in another thread and return a concurrent.futures Future objects that can be used to wait for the result. For example:

import asyncio
import concurrent.futures

def sync_func(loop):
    # This function is not a coroutine and cannot be run by the event loop
    # It takes the event loop as an argument and submits a coroutine to it
    future = asyncio.run_coroutine_threadsafe(async_func(), loop) # This returns a concurrent.futures.Future object
    result = future.result() # This blocks until the coroutine is done and returns the result
    print(result)

async def async_func():
    # This function is a coroutine and can be run by the event loop
    await asyncio.sleep(1) # This pauses the execution for 1 second
    return "Hello from async_func"

async def main():
    # This is the main coroutine of the program
    loop = asyncio.get_running_loop() # This gets the running event loop
    executor = concurrent.futures.ThreadPoolExecutor() # This creates a thread pool executor
    await loop.run_in_executor(executor, sync_func, loop) # This runs the sync_func in a thread and passes the event loop as an argument
    print("Hello from main")

asyncio.run(main()) # This runs the main coroutine and creates an event loop

Use a loop if you need to run a synchronous function from a coroutine.run_in_executor()

This function will run the synchronous function in a thread or process pool executor and return an asyncio.Future objects that can be awaited for the result. For example:

import asyncio
import concurrent.futures

def sync_func():
    # This function is not a coroutine and cannot be run by the event loop
    return "Hello from sync_func"

async def main():
    # This is the main coroutine of the program
    loop = asyncio.get_running_loop() # This gets the running event loop
    executor = concurrent.futures.ThreadPoolExecutor() # This creates a thread pool executor
    result = await loop.run_in_executor(executor, sync_func) # This runs the sync_func in a thread and returns an asyncio.Future object
    print(result)

asyncio.run(main()) # This runs the main coroutine and creates an event loop

Following these guidelines, you can avoid the “DeprecationWarning: there is no current event loop” error and write more robust and reliable asynchronous programs in Python.

FAQs

Conclusion

The “DeprecationWarning: there is no current event loop” is a warning message that indicates that the code is trying to access an event loop that does not exist or has not been set for the current thread. This warning was introduced in Python 3.10 to discourage using the asyncio.get_event_loop() function, which can cause confusion and unexpected behavior.

To resolve this error, it is recommended to follow the best practices for using event loops in Python, such as using asyncio.run() to run the main coroutine of the program and create an event loop, using asyncio.get_running_loop() to get the running event loop in a coroutine, or passing the event loop explicitly as an argument to functions that need it. Following these guidelines, you can avoid the “DeprecationWarning: there is no current event loop” error and write more robust and reliable asynchronous programs in Python.

Reference

1. DeprecationWarning
2. asyncio.run

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