How to Add Custom Validation In Fastapi?

To add custom validation in FastAPI, you can create a custom Field class that inherits from the Depends class provided by FastAPI. Inside this custom Field class, you can define your custom validation logic by overriding the __call__ method.

Within the __call__ method, you can perform any validation checks you need and raise a HTTPException with an appropriate status code and error message if the data does not pass the validation. You can then use this custom Field class as a dependency in your route operation functions by including it in the dependencies parameter.

By using custom validation in FastAPI, you can easily implement complex validation logic for your API endpoints and ensure that incoming data meets your specific requirements before processing it further.

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What are the benefits of using custom validation in FastAPI?

1. Increased security: Custom validation allows you to define specific validation rules for your API endpoints, ensuring that only valid and expected data is accepted. This can help prevent malicious or incorrect data from being processed by your application.
2. Improved error handling: By defining custom validation logic, you can provide more detailed and user-friendly error messages when validation fails. This can help users understand why their input was rejected and how they can correct it.
3. Enhanced data integrity: Custom validation can help maintain data integrity by ensuring that only valid data is stored in your database. This can prevent data corruption or inconsistencies in your application.
4. Flexibility: Custom validation allows you to define unique validation rules tailored to your specific application requirements. This gives you more control over the data that is accepted by your API endpoints.
5. Code reusability: Custom validation logic can be reused across multiple endpoints, reducing code duplication and making it easier to maintain and update validation rules in the future.

What is the role of dependency injection in custom validation functions in FastAPI?

Dependency injection in FastAPI allows you to inject custom dependencies, such as custom validation functions, into your endpoints. This means that you can define your own validation logic and have it applied to specific endpoints without having to repeat the same code multiple times.

By using dependency injection, you can separate concerns and keep your code clean and organized. You can define your custom validation functions as dependencies and then inject them into your endpoint functions, allowing you to easily reuse and manage your validation logic.

Overall, dependency injection in FastAPI enables you to create more modular and flexible validation functions that can be easily integrated into your API endpoints.

How to define custom validators for Pydantic models in FastAPI?

To define custom validators for Pydantic models in FastAPI, you can use Pydantic's validator decorator. Here is an example of how to define a custom validator for a Pydantic model in FastAPI:

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from pydantic import BaseModel, validator
from fastapi import FastAPI

class User(BaseModel):
    username: str
    password: str

    @validator('username')
    def username_validator(cls, v):
        if len(v) < 5:
            raise ValueError('Username must be at least 5 characters long')
        return v

    @validator('password')
    def password_validator(cls, v):
        if len(v) < 8:
            raise ValueError('Password must be at least 8 characters long')
        return v

app = FastAPI()

@app.post('/create_user')
async def create_user(user: User):
    return {'username': user.username, 'password': user.password}

In this example, the User model has two fields: username and password. Two custom validators (username_validator and password_validator) are defined using the @validator decorator. These validators check that the username is at least 5 characters long and the password is at least 8 characters long. If the validation fails, a ValueError is raised.

When a POST request is made to the /create_user endpoint with a JSON payload that matches the User model, FastAPI will automatically validate the request body using the custom validators. If the validation passes, the handler function will be called with the validated data.

How to add custom validation middleware in FastAPI?

To add custom validation middleware in FastAPI, you can create a custom middleware class that inherits from Middleware and implement the from_queryset method. Below is an example code snippet to demonstrate how to add custom validation middleware in FastAPI:

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from fastapi import FastAPI, Request, Response
from fastapi.middleware import Middleware

class CustomValidationMiddleware:
    async def __call__(self, request: Request, response: Response, call_next):
        # Custom validation logic
        if not request.headers.get('Authorization'):
            return Response(status_code=401, content="Unauthorized")
        
        return await call_next(request)

app = FastAPI()

custom_validation_middleware = CustomValidationMiddleware()

app.add_middleware(Middleware, custom_validation_middleware)

@app.get("/")
async def read_root():
    return {"Hello": "World"}

In this example, we have created a CustomValidationMiddleware class that checks if the request has an Authorization header. If the header is missing, it returns a 401 Unauthorized response. We then add the custom validation middleware to the FastAPI app using the add_middleware method.

You can customize the CustomValidationMiddleware class to add any custom validation logic as needed for your API.