Do dependencies slow down the application?

No. FastAPI's dependency resolution engine is incredibly fast. In fact, because it can resolve independent sub-dependencies concurrently (if they are async), it can actually be faster than executing functions sequentially.

Can I use a class as a dependency?

Yes! If you pass a Python class to `Depends(MyClass)`, FastAPI will automatically instantiate the class using the incoming request data and inject the newly created object.

What if multiple endpoints use the same dependency?

By default, if the same dependency is called multiple times within a single HTTP request (e.g. via sub-dependencies), FastAPI caches the result. It only executes the dependency once per request.

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Tutorials

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Core Dependency Injection

Master FastAPI's Dependency Injection system. Learn how to use Depends(), share pagination logic, inject database sessions safely, build dependency trees, and override dependencies for automated testing.

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