pytest-asyncio has a closed event loop, but only when running all tests

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06-10-2024

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pytest-asyncio: Closed Event Loop Mystery When Running All Tests

Problem: You're using pytest-asyncio to write asynchronous tests, but you encounter a mysterious error: "RuntimeError: Event loop is closed". This error only happens when you run all your tests, not when you run individual test functions.

Rephrased: Imagine you have a group of friends who love playing games, and you want to organize a tournament. You have a special game board (the event loop) that everyone needs to use. If you let everyone play one game at a time (run individual tests), everything works smoothly. However, when everyone tries to play at once (run all tests), the board suddenly shuts down, leaving everyone in the middle of their games.

Scenario:

Let's say you have a simple test file:

import asyncio
import pytest

@pytest.mark.asyncio
async def test_async_function():
    await asyncio.sleep(0.1)
    assert True

This test uses pytest-asyncio to mark a simple asynchronous function. When running this test individually, it works perfectly. However, when you run the entire test file, you get the dreaded "RuntimeError: Event loop is closed" error.

Analysis:

This issue arises due to how pytest-asyncio handles event loops during test runs. By default, pytest-asyncio creates a new event loop for each test function. However, when running all tests, pytest-asyncio tries to reuse a single event loop across multiple test functions. This can lead to conflicts, especially if a test function doesn't properly clean up after itself or if it closes the event loop prematurely.

Possible Solutions:

1. Explicitly Create and Close Event Loops: Instead of relying on pytest-asyncio's default behavior, you can explicitly create and close your own event loop within each test function. This ensures that each test operates in a clean environment.

   @pytest.mark.asyncio
   async def test_async_function():
       loop = asyncio.new_event_loop()
       asyncio.set_event_loop(loop) 
       try:
           await asyncio.sleep(0.1)
           assert True
       finally:
           loop.close()
   

2. Use pytest.fixture for Event Loop Management: A more elegant approach is to utilize a pytest fixture to manage the event loop across your test functions. You can set up the loop in the fixture and ensure it's properly closed after each test.

   @pytest.fixture
   def event_loop():
       loop = asyncio.new_event_loop()
       yield loop
       loop.close()
   
   @pytest.mark.asyncio
   async def test_async_function(event_loop):
       asyncio.set_event_loop(event_loop)
       await asyncio.sleep(0.1)
       assert True 
   

3. Check for Proper Event Loop Cleanup: Ensure that your asynchronous code doesn't close the event loop prematurely or leaves tasks running after a test has completed. Use asyncio.gather to wait for all tasks to finish before ending your tests.

Additional Value:

• Consider the Impact of pytest-asyncio's Behavior: Understand that the "Run all tests" mode might be slightly less performant than running individual tests due to the overhead of creating and closing event loops for each test. However, it generally provides a more isolated and reliable testing environment.

• Leverage pytest-asyncio's Advanced Features: Explore features like pytest_asyncio.MonkeyPatch, which allows you to patch asynchronous functions during tests for increased control over your test environment.

References:

• pytest-asyncio Documentation
• pytest Documentation

By understanding the cause of this common issue and implementing these solutions, you can ensure your pytest-asyncio tests run reliably whether you're executing individual tests or the entire suite.