Handling exceptions

Introduction

FastAPI provides a straightforward and efficient mechanism to handle exceptions which allows you to manage errors in a clean, pythonic way. In this tutorial, we will explore how to handle exceptions using FastAPI.

What is an Exception?

An exception is an event that disrupts the normal flow of your program. It's a Python object that represents an error. When a Python script encounters a situation that it cannot cope with, it raises an exception.

FastAPI's Exception Handlers

FastAPI provides the HTTPException class to define HTTP exceptions. It's a simple way to return responses with HTTP error codes and a customizable detail.

Here's a basic example:

from fastapi import FastAPI, HTTPException

app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: str):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item": items[item_id]}

In this example, if the item_id is not found in our items dictionary, an HTTPException is raised with a status code of 404 and a detail "Item not found".

Custom Exception Handlers

FastAPI allows you to define your own exception handlers using the app.exception_handler decorator. This is useful if you want to customize the response format, or handle exceptions in a specific way.

Here's how you can create a custom exception handler:

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

app = FastAPI()

@app.exception_handler(HTTPException)
async def custom_http_exception_handler(request: Request, exc: HTTPException):
    return JSONResponse(
        status_code=exc.status_code,
        content={"custom_error": f"Oops! {exc.detail} occurred."},
    )

In this example, we've defined a custom handler for HTTPException. It returns a JSONResponse with a custom message and the status code from the exception.

Using FastAPI's Starlette Exceptions

FastAPI is built on Starlette for the web parts. This means you can use Starlette's built-in RequestValidationError and HTTPException in your FastAPI application.

Here's an example:

from fastapi import FastAPI, HTTPException
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()

@app.exception_handler(StarletteHTTPException)
async def http_exception_handler(request, exc):
    return PlainTextResponse(str(exc.detail), status_code=exc.status_code)

In this example, we're using Starlette's HTTPException to raise an HTTP exception and we're defining a custom exception handler for it.

Conclusion

FastAPI provides a powerful and flexible way to handle exceptions in your API. It allows for easy customization of error responses and integrates well with its underlying Starlette framework.

Remember, good error handling can greatly improve the robustness of your application and the efficiency of resolving issues.

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I hope this tutorial is helpful in your FastAPI journey. Happy learning!