前言

通常我们可以通过 raise 抛出一个 HTTPException 异常，请求参数不合法会抛出RequestValidationError 异常，这是最常见的2种异常。

`HTTPException` 异常

向客户端返回 HTTP 错误响应，可以使用 raise 触发 HTTPException。

from fastapi import FastAPI, HTTPException

app = FastAPI()


@app.get("/path/{name}")  
async def read_unicorn(name: str):  
    if name == "yoyo":  
        raise HTTPException(404, detail=f"name: {name} not found")  
    return {"path_name": name}

默认情况下返回json格式

HTTP/1.1 404 Not Found
date: Wed, 27 Sep 2023 02:07:07 GMT
server: uvicorn
content-length: 22
content-type: application/json

{"detail":"Not Found"}

覆盖默认的`HTTPException` 异常

查看HTTPException 异常相关源码

from starlette.exceptions import HTTPException as StarletteHTTPException  
  
  
class HTTPException(StarletteHTTPException):  
    def __init__(  
        self,  
        status_code: int,  
        detail: Any = None,  
        headers: Optional[Dict[str, Any]] = None,  
    ) -> None:  
        super().__init__(status_code=status_code, detail=detail, headers=headers)

HTTPException 异常是继承的 starlette 包里面的 HTTPException
覆盖默认异常处理器时需要导入 from starlette.exceptions import HTTPException as StarletteHTTPException，并用 @app.excption_handler(StarletteHTTPException) 装饰异常处理器。

from fastapi import FastAPI, Request  
from fastapi.exceptions import HTTPException  
from fastapi.responses import PlainTextResponse, JSONResponse  
from starlette.exceptions import HTTPException as StarletteHTTPException  
  
  
app = FastAPI()  
  
  
# # 捕获 HTTPException 异常  
@app.exception_handler(StarletteHTTPException)  
def http_error(request, exc):  
    print(exc.status_code)  
    print(exc.detail)  
    # return JSONResponse({'error_msg': exc.detail}, status_code=exc.status_code)  
    return PlainTextResponse(content=exc.detail, status_code=exc.status_code)  
  
  
@app.get("/path/{name}")  
async def read_unicorn(name: str):  
    if name == "yoyo":  
        raise HTTPException(404, detail=f"name: {name} not found")  
    return {"path_name": name}

这样原来的 HTTPException 返回 json 格式，现在改成返回text/plain 文本格式了。

HTTP/1.1 404 Not Found
date: Wed, 27 Sep 2023 07:24:58 GMT
server: uvicorn
content-length: 20
content-type: text/plain; charset=utf-8

name: yoyo not found

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。

from fastapi import FastAPI, HTTPException
from fastapi.exceptions import RequestValidationError
from fastapi.responses import PlainTextResponse
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    return PlainTextResponse(str(exc), status_code=400)


@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id == 3:
        raise HTTPException(status_code=418, detail="Nope! I don't like 3.")
    return {"item_id": item_id}

访问 /items/foo，可以看到以下内容替换了默认 JSON 错误信息：

{
    "detail": [
        {
            "loc": [
                "path",
                "item_id"
            ],
            "msg": "value is not a valid integer",
            "type": "type_error.integer"
        }
    ]
}

以下是文本格式的错误信息：

HTTP/1.1 400 Bad Request
date: Wed, 27 Sep 2023 07:30:38 GMT
server: uvicorn
content-length: 103
content-type: text/plain; charset=utf-8

1 validation error for Request
path -> item_id
  value is not a valid integer (type=type_error.integer)

RequestValidationError 源码分析

RequestValidationError 相关源码

class RequestValidationError(ValidationError):  
    def __init__(self, errors: Sequence[ErrorList], *, body: Any = None) -> None:  
        self.body = body  
        super().__init__(errors, RequestErrorModel)

使用示例

from fastapi import FastAPI, Request, status
from fastapi.encoders import jsonable_encoder
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse
from pydantic import BaseModel

app = FastAPI()


@app.exception_handler(RequestValidationError)  
async def validation_exception_handler(request: Request, exc: RequestValidationError):  
    print(exc.json())  
    print(exc.errors())  
    print(exc.body)   # 请求body  
    return JSONResponse(  
        status_code=400,  
        content=jsonable_encoder({"detail": exc.errors(), "body": exc.body}),  
    )  
  
  
class Item(BaseModel):  
    title: str  
    size: int  
  
  
@app.post("/items/")  
async def create_item(item: Item):  
    return item

现在试着发送一个无效的 item，例如：

{
  "title": "towel",
  "size": "XL"
}

运行结果

HTTP/1.1 400 Bad Request
date: Wed, 27 Sep 2023 07:51:36 GMT
server: uvicorn
content-length: 138
content-type: application/json

{"detail":[{"loc":["body","size"],"msg":"value is not a valid integer","type":"type_error.integer"}],"body":{"title":"towel","size":"XL"}}