How to Fix RuntimeError: Event Loop is Closed When Using Pytest with FastAPI

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Summary: Learn how to resolve the common `RuntimeError: Event Loop is Closed` error when running Pytest with FastAPI, ensuring smoother asynchronous testing in your Python applications.
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How to Fix RuntimeError: Event Loop is Closed When Using Pytest with FastAPI

If you have been working with FastAPI and using Pytest for testing, you might have encountered the dreaded RuntimeError: Event loop is closed. This problem often arises when dealing with asynchronous code, but don't worry — it's manageable with a few tweaks.

Understanding the RuntimeError

The RuntimeError: Event loop is closed is an error that points to issues within event loops, commonly seen within asynchronous environments. In FastAPI, which relies heavily on asynchronous I/O operations, this error might surface if the event loop you are working with gets closed prematurely.

Quick Fixes

Here are a couple of methods to handle this error effectively:

Using the pytest-asyncio Plugin
One of the simplest approaches is to use the pytest-asyncio plugin. This plugin allows you to run and test asynchronous code seamlessly with Pytest.

Installation

To install pytest-asyncio, run:

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Usage

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Overwrite the Default Event Loop

Another method entails overriding the default event loop policy. This is beneficial if the event loop gets cleaned up before the test runner is done.

Overwrite via Setup

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Using a Custom Event Loop in Test Functions

Alternatively, you can start a new event loop within each test function:

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Conclusion

The RuntimeError: Event loop is closed error can be a bit daunting, especially when you're new to asynchronous programming with FastAPI and Pytest. By implementing the above fixes, you will ensure that your event loop stays open during the entire test execution, making your testing process smoother and more efficient.

Feel free to choose the method that best suits your need and ensures a seamless testing experience. Happy coding!