不过，也可以使用自定义处理器覆盖默认异常处理器。

### 覆盖请求验证异常

请求中包含无效数据时，**FastAPI** 内部会触发 `RequestValidationError`。

该异常也内置了默认异常处理器。

覆盖默认异常处理器时需要导入 `RequestValidationError`，并用 `@app.excption_handler(RequestValidationError)` 装饰异常处理器。

这样，异常处理器就可以接收 `Request` 与异常。

{* ../../docs_src/handling_errors/tutorial004.py hl[2,14:16] *}

访问 `/items/foo`，可以看到默认的 JSON 错误信息：

```JSON
{

 * under the License.
 */
package org.apache.maven.exception;

/**
 * Transform an exception into useful end-user message.
 *
 * @since 3.0-alpha-3
 */
public interface ExceptionHandler {
    ExceptionSummary handleException(Throwable e);

from fastapi import FastAPI, HTTPException
from fastapi.exception_handlers import (
    http_exception_handler,
    request_validation_exception_handler,
)
from fastapi.exceptions import RequestValidationError
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()


@app.exception_handler(StarletteHTTPException)
async def custom_http_exception_handler(request, exc):
    print(f"OMG! An HTTP error!: {repr(exc)}")

Ben Beasley <******@****.***> 1758043308 +0100

app.mount(path="/sub", app=sub_app)


@app.exception_handler(RequestValidationError)
@sub_app.exception_handler(RequestValidationError)
async def request_validation_handler(request: Request, exc: RequestValidationError):
    captured_exception.capture(exc)
    raise exc


@app.exception_handler(ResponseValidationError)
@sub_app.exception_handler(ResponseValidationError)

Let's say you have a custom exception `UnicornException` that you (or a library you use) might `raise`.

And you want to handle this exception globally with FastAPI.

You could add a custom exception handler with `@app.exception_handler()`:

{* ../../docs_src/handling_errors/tutorial003_py39.py hl[5:7,13:18,24] *}

Here, if you request `/unicorns/yolo`, the *path operation* will `raise` a `UnicornException`.

Y quieres manejar esta excepción globalmente con FastAPI.

Podrías agregar un manejador de excepciones personalizado con `@app.exception_handler()`:

{* ../../docs_src/handling_errors/tutorial003_py39.py hl[5:7,13:18,24] *}

Aquí, si solicitas `/unicorns/yolo`, la *path operation* lanzará un `UnicornException`.

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)


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc: RequestValidationError):
    message = "Validation errors:"

    @Inject
    private LifeCyclePluginAnalyzer lifeCyclePluginAnalyzer;

    @Inject
    private LifecycleTaskSegmentCalculator lifeCycleTaskSegmentCalculator;

    @Inject
    private ExceptionHandler exceptionHandler;

    protected String getProjectsDirectory() {
        return "src/test/projects/lifecycle-executor";
    }

    @Test
    void testCreation() throws Exception {

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


class UnicornException(Exception):
    def __init__(self, name: str):
        self.name = name


app = FastAPI()


@app.exception_handler(UnicornException)
async def unicorn_exception_handler(request: Request, exc: UnicornException):
    return JSONResponse(
        status_code=418,