如何用FastAPI解析SendGrid的原始multipart/form-data Webhook？

我想用FastAPI接收SendGrid的入站解析Webhook，该Webhook以原始multipart/form-data格式传递数据。我的代码能接收到负载，但解析时会出现422 Unprocessable Entity错误（Pydantic验证失败）。

请问是否可以直接通过FastAPI解析这类数据？如果需要手动解析再转换格式传入FastAPI，希望提供示例代码。

最小可复现示例 #

FastAPI接收代码 #

from typing import Any
from fastapi import FastAPI, Form

app = FastAPI()

@app.post("/inbound_email")
async def inbound_email(
    headers: Any = Form(...),
    subject: Any = Form(...),
    ):
    try:
        print(headers)
        print(subject)
    except Exception as e:
        print(f"inbound_email exception: {e}")

if __name__ == "__main__":
    import uvicorn
    uvicorn.run("min_reproducible_example:app", host="127.0.0.1", port=54200, reload=True)

模拟SendGrid Webhook的curl测试命令 #

curl --location 'https://myurlhere.ngrok-free.app/inbound_email' \
--data '--xYzZY
Content-Disposition: form-data; name="headers"

Content-Type: multipart/alternative; boundary="00000000000021b9ea0605bbd8ac"

--xYzZY
Content-Disposition: form-data; name="subject"

Inbound test 5
--xYzZY--
'

错误输出 #

应用日志：

"POST /inbound_email HTTP/1.1" 422 Unprocessable Entity

Postman返回的详细错误：

{
    "detail": [
        {
            "type": "missing",
            "loc": [
                "body",
                "headers"
            ],
            "msg": "Field required",
            "input": null
        }
    ]
}

尝试直接解析Request的代码及结果 #

from fastapi import FastAPI, Request

app = FastAPI()

@app.post("/inbound_email")
async def inbound_email(
    request: Request,
    ):
    try:
        raw_body = await request.body()
        print(raw_body.decode("utf-8"))
        form_data = await request.form()
        body_headers = form_data.get("headers")
        subject = form_data.get("subject")
        print("body_headers:", body_headers)
        print("subject:", subject)
    except Exception as e:
        print(f"inbound_email exception: {e}")

if __name__ == "__main__":
    import uvicorn
    uvicorn.run("min_reproducible_example:app", host="127.0.0.1", port=54200, reload=True)

终端输出：

--xYzZY
Content-Disposition: form-data; name="headers"

Content-Type: multipart/alternative; boundary="00000000000021b9ea0605bbd8ac"

--xYzZY
Content-Disposition: form-data; name="subject"

Inbound test 5
--xYzZY--

body_headers: None
subject: None

补充说明 #

• 通过Postman发送请求时，以raw格式传递正文文本，使用默认请求头，也曾尝试显式指定格式、边界符等请求头，均无效。
• 其他格式（包括Postman的form-data）的POST请求能正常被FastAPI解析，仅这种原始文本格式的multipart/form-data无法解析。

内容的提问来源于stack exchange，提问作者FlightPlan