FastAPI POST请求报错：参数不匹配与method对象不可下标问题排查

问题描述 #

向FastAPI后端发送POST请求时，先后触发两个错误：

1. 初始无参数的test函数报错：test() takes 0 positional arguments but 1 was given
2. 添加request参数后，报错：TypeError: 'method' object is not subscriptable

前端代码 #

import requests

data = {"foo": "bar"}

r = requests.post("http://127.0.0.1:8000/test", json=data)

print(r.text)

后端代码（出错版本） #

import requests
from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

@app.route('/test', methods=['POST'])
def test(request):
    print(request.form['foo'])
    return 'thisIsResponse'

错误堆栈信息 #

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/uvicorn/protocols/http/httptools_impl.py", line 375, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/usr/lib/python3/dist-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/fastapi/applications.py", line 271, in __call__
    await super().__call__(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/applications.py", line 118, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/middleware/cors.py", line 84, in __call__
    await self.app(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
    raise exc
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "/home/spree/.local/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__
    raise e
  File "/home/spree/.local/lib/python3.10/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/routing.py", line 706, in __call__
    await route.handle(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/routing.py", line 276, in handle
    await self.app(scope, receive, send)
  File "/home/spree/.local/lib/python3.10/site-packages/starlette/routing.py", line 66, in app
    response = await func(request)
  File "/home/spree/spreeTv/./ods_info.py", line 30, in test
    print(request.form['foo'])
TypeError: 'method' object is not subscriptable

错误原因 #

1. 第一个错误：使用@app.route装饰器时，FastAPI（底层基于Starlette）会自动向路由函数传入Request对象作为参数，无参数的函数无法接收该参数，因此触发参数不匹配错误。
2. 第二个错误：
   • request.form是异步方法，必须通过await调用才能获取表单数据，直接用下标访问会触发“方法不可下标”的错误；
   • 前端发送的是JSON格式数据（Content-Type: application/json），而非表单编码数据，即使正确调用request.form()也无法获取到数据。

解决方法 #

方法一：FastAPI原生请求体解析（推荐） #

利用FastAPI的Pydantic模型自动解析JSON请求体，代码更简洁且自带数据校验：

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
from pydantic import BaseModel

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

# 定义请求体结构模型
class RequestData(BaseModel):
    foo: str

# 使用FastAPI专属的@app.post装饰器
@app.post('/test')
def test(data: RequestData):
    print(data.foo)
    return 'thisIsResponse'

方法二：手动处理Request对象（特殊场景适用） #

若需直接操作Request对象，需将函数定义为异步函数，并通过await request.json()获取JSON数据：

from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

@app.post('/test')
async def test(request: Request):
    json_data = await request.json()
    print(json_data['foo'])
    return 'thisIsResponse'

注意事项 #

• 前端代码无需修改，已正确使用json=data发送JSON请求；
• 优先使用方法一，FastAPI会自动生成API文档、数据校验等功能，符合框架设计规范。

内容的提问来源于stack exchange，提问作者seriously