Dependencies - Path Operation Decorators

Dependencies - Path Operation Decorators

🎯 What You'll Learn

- Learn how to add dependencies to path operation decorators

- Understand when to use decorator-level dependencies

- Implement dependencies that don't need to return values to the function

📚 Theory

In some cases you don't really need the return value of a dependency inside your path operation function.

Or the dependency doesn't return a value.

But you still need it to be executed/solved.

For those cases, instead of declaring a path operation function parameter with Depends, you can add a list of dependencies to the path operation decorator.

Add dependencies to the Path Operation Decorator

The path operation decorator receives an optional argument dependencies.

It should be a list of Depends():

[Code Example]

These dependencies will be executed/solved the same way as normal dependencies. But their value (if they return any) won't be passed to your path operation function.

Tip: Some editors check for unused function parameters, and show them as errors. Using these dependencies in the path operation decorator you can make sure they are executed while avoiding editor/tooling errors.

It might also help avoid confusion for new developers that see an unused parameter in your code and could think it's unnecessary.

Dependencies Errors and Return Values

You can use the same dependency functions you use normally.

Dependency Requirements

They can declare request requirements (like headers) or other sub-dependencies:

[Code Example]

Raise Exceptions

These dependencies can raise exceptions, the same as normal dependencies:

[Code Example]

Return Values

And they can return values or not, the values won't be used.

So, you can reuse a normal dependency (that returns a value) you already use somewhere else, and even though the value won't be used, the dependency will be executed.

🔧 Key Concepts

Decorator Dependencies

Add dependencies to the decorator using a list of Depends() calls:

[Code Example]

Execution Without Return Values

Dependencies in decorators:

- Execute normally and can raise exceptions

- Can access request data (headers, query params, etc.)

- Return values are ignored (not passed to the function)

- Still validate requirements and sub-dependencies

Common Use Cases

- Security checks without needing user data in function

- Header validation for API keys or custom headers

- Request logging or monitoring

- Rate limiting or quota checks

💡 Best Practices

- Use decorator dependencies when you don't need the return value

- Combine with function parameters when you need some dependency values

- Reuse existing dependencies - the same dependency function can be used in both places

- Handle exceptions - dependencies can still raise HTTPExceptions

- Editor-friendly - avoids unused parameter warnings

🔗 Additional Resources

- FastAPI Dependencies Guide

- Security Dependencies