How to implement error handling and custom exceptions in FastAPI

How to implement error handling and custom exceptions in FastAPI

Introduction:
FastAPI is a modern web framework based on Python. Its high performance and rapid development capabilities make it more and more popular in the development field. The more popular it becomes. In actual applications, errors and exceptions are often encountered. This article will introduce how to implement error handling and custom exceptions in FastAPI to help developers better handle and manage error situations in applications.

FastAPI error handling:
FastAPI provides a built-in exception handling mechanism. By using exception handlers or decorators, we can capture and handle errors in the application. The following is a basic error handling example:

from fastapi import FastAPI

app = FastAPI()

@app.exception_handler(Exception)
async def validation_exception_handler(request, exc):
    return JSONResponse(status_code=400, content={"message": "Bad request"})

@app.get("/users/{user_id}")
async def read_user(user_id: int):
    if user_id <= 0:
        raise Exception("Invalid user id")
    return {"user_id": user_id}

In the above example, we use the @app.exception_handler decorator to define a global exception handler. It accepts two parameters, the first parameter is the exception type to be handled, and the second parameter is a callback function used to handle the exception. In this example, we catch all Exception exceptions and return a JSONResponse with a custom error message.

When we access the /users/{user_id} route, if the incoming user_id is less than or equal to 0, a custom exception will be thrown. Through the global exception handler defined above, we can catch and handle this exception and return a JSONResponse with an error message.

Custom exceptions:
In addition to using the built-in exception types, we can also customize exception types to better distinguish and handle different types of errors. Here is an example of a custom exception:

class InvalidUserIdException(Exception):
    def __init__(self, user_id: int):
        self.user_id = user_id
        super().__init__("Invalid user id")

@app.get("/users/{user_id}")
async def read_user(user_id: int):
    if user_id <= 0:
        raise InvalidUserIdException(user_id)
    return {"user_id": user_id}

In the above example, we defined a custom exception class named InvalidUserIdException, which inherits from Exceptionkind. We also accept a user_id parameter in the constructor to display the specific user ID in the exception message. In the routing processing function, when user_id is less than or equal to 0, we throw this custom exception.

When we run the application again, we see the same results as before. This is because we did not define a specific exception handler for this custom exception. In order to catch and handle custom exceptions, we can add a new exception handler:

@app.exception_handler(InvalidUserIdException)
async def invalid_user_id_exception_handler(request, exc):
    return JSONResponse(status_code=400, content={"message": str(exc)})

In the above example, we have added a new exception handler to catch InvalidUserIdExceptionabnormal. It has the same structure as the previous global exception handler, but the processing logic is different. In this handler, we convert the exception message to a string and return a JSONResponse with the error message.

In this way, when we access the /users/{user_id} route, if the incoming user_id is less than or equal to 0, InvalidUserIdException will be thrown abnormal. With a specific exception handler, we can catch and handle this exception and return a JSONResponse with a custom error message.

Summary:
By using FastAPI’s error handling mechanism, we can better handle and manage error situations in our applications. We can use a global exception handler to catch and handle all exceptions, or we can define specific exception handlers to handle specific exception types. At the same time, we can also customize exception classes to better distinguish and handle different types of errors, thereby improving the readability and maintainability of the code.

Reference link:

• FastAPI official documentation: https://fastapi.tiangolo.com/tutorial/handling-errors/
• Python official documentation: https: //docs.python.org/3/tutorial/errors.html

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