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Discover how to use `FastAPI` class dependencies with async operations effectively. Learn to handle `async` initialization without compromising functionality.
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This video is based on the question https://stackoverflow.com/q/65243587/ asked by the user 'Vlad' ( https://stackoverflow.com/u/186099/ ) and on the answer https://stackoverflow.com/a/65247387/ provided by the user 'alex_noname' ( https://stackoverflow.com/u/13782669/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Leveraging FastAPI for Async Class Dependencies

FastAPI is an incredible framework for building APIs in Python, known for its speed and ease of use. One of its standout features is dependency injection, which allows developers to manage dependencies efficiently. However, when it comes to async operations within class-based dependencies, many developers often face challenges.

Understanding the Problem

When we define a standard dependency in FastAPI, we can use either a regular function (def) or an asynchronous function (async def). While FastAPI handles both scenarios gracefully, class-based dependencies introduce complexities, particularly when we need to perform asynchronous operations during their initialization.

Class definitions typically require a synchronous _init_ method. This limitation can mislead developers into thinking that they cannot use async operations when initializing class dependencies. So, how can we overcome this?

Solution: Using Async Sub-dependency

While the _init_ method itself cannot be async, FastAPI allows us to work around this by utilizing a strategy where async operations are turned into sub-dependencies. Let's break this down into manageable steps.

Step 1: Define Your Async Dependency

First, define your asynchronous dependency as a standalone function. This function can then be utilized in the class without disrupting the synchronous flow of the _init_ method.

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create a Class with Sync Initialization

Next, define your class where the constructor (__init__) accepts an instance of your async dependency. By using Depends(), FastAPI will manage the async operation for you:

[[See Video to Reveal this Text or Code Snippet]]

Putting It All Together

In the above example:

The method async_dep() simulates an asynchronous operation (like an API call or database query) using asyncio.sleep.

The CommonQueryParams class initializes with a value fetched from the async_dep() asynchronously without needing the _init_ method to be async.

When FastAPI calls this dependency, it will correctly await the result of async_dep() before passing it to the class.

Conclusion

By structuring your dependencies wisely, you can effectively utilize both FastAPI’s class-based dependency injection and async functionalities, leading to cleaner, more maintainable code. So the next time you’re building a FastAPI application that requires async capability within class dependencies, remember to leverage sub-dependencies as shown above. Happy coding with FastAPI!