Custom Exception Handling in FastAPI

Key Concepts

Custom Exception Handling in FastAPI allows you to define and manage exceptions specific to your application. This feature enhances the robustness and clarity of your API by providing tailored error responses. Here are the key concepts:

• Custom Exceptions: User-defined exceptions that extend the base Exception class.
• Exception Handlers: Functions that handle specific exceptions and return custom responses.
• Global Exception Handling: Handling exceptions globally across the application.
• HTTPException: A built-in FastAPI exception that simplifies returning HTTP errors.

1. Custom Exceptions

Custom exceptions are user-defined exceptions that extend the base Exception class. They allow you to create specific error types that can be raised and handled within your application.

Example:

class InvalidUserException(Exception):
    def __init__(self, user_id: int, message: str = "Invalid user ID"):
        self.user_id = user_id
        self.message = message
        super().__init__(self.message)
    

2. Exception Handlers

Exception handlers are functions that handle specific exceptions and return custom responses. They are registered using the @app.exception_handler() decorator.

Example:

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(InvalidUserException)
async def invalid_user_exception_handler(request: Request, exc: InvalidUserException):
    return JSONResponse(
        status_code=400,
        content={"message": f"Oops! User {exc.user_id} is invalid. {exc.message}"},
    )
    

3. Global Exception Handling

Global exception handling allows you to handle exceptions that are not caught by specific exception handlers. This is useful for managing unexpected errors.

Example:

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

app = FastAPI()

@app.exception_handler(Exception)
async def global_exception_handler(request: Request, exc: Exception):
    return JSONResponse(
        status_code=500,
        content={"message": "An unexpected error occurred. Please try again later."},
    )
    

4. HTTPException

HTTPException is a built-in FastAPI exception that simplifies returning HTTP errors. It allows you to raise exceptions with specific status codes and messages.

Example:

from fastapi import FastAPI, HTTPException

app = FastAPI()

@app.get("/users/{user_id}")
async def read_user(user_id: int):
    if user_id < 0:
        raise HTTPException(status_code=400, detail="User ID must be a positive integer")
    return {"user_id": user_id}
    

Analogies

Think of custom exceptions as specific error codes in a manual. Just as a manual has detailed instructions for different errors, custom exceptions provide specific error messages for different scenarios in your application.

Exception handlers are like customer service representatives who handle specific issues. They know how to address and resolve particular problems, providing tailored responses.

Global exception handling is like a general support team that handles any issue not covered by specialized representatives. They provide a fallback solution for unexpected problems.

HTTPException is like a standardized error message system. It ensures that all errors follow a consistent format, making it easier to understand and resolve issues.

By mastering custom exception handling in FastAPI, you can create more robust and user-friendly APIs, ensuring that errors are handled gracefully and informatively.