How to parse the request data using Form and still log the Request body?

[gor-bit]

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Example Code

from fastapi import Request, FastAPI, Form

app = FastAPI()

@app.post("/post_name")
async def parse_data(request: Request, first_name: str = Form(...), last_name: str = Form(...)):
    data_body = await request.body()
    logging.info(data_body)  
    logging.info(first_name)
    logging.info(last_name)
    return first_name + last_name

Description

I have an endpoint /post_name for users to send their first_name and last_name as form data.

This can be simulated using a curl command:

curl -d first_name=Sabrina -d last_name=Alami -X POST website.com/post_name

However, occasionally I may receive people who accidentally send their data using second_name instead of last_name. Or they may include another parameter such a middle_name due to the common tradition of my country which results in an error.

For requests which are successful, I am able to log the first_name and last_name fields easily. But I want to be able to log the body of all requests, including those which result in an error. Hence I want to log the body of all requests received so I don't lose this information.

The above code snippet should do this, but gives me the following error whenever I receive a request:

RuntimeError: Stream consumed

What am I doing wrong when trying to access request.body()?

Thank you.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.68.1

Python Version

Python 3.8.10

Additional Context

No response

[frankie567]

Use the .form method instead of .body, it'll directly give you a parsed version of the form payload. Since internally FastAPI already called it to parse first_name and last_name, you don't consume the stream twice, the result is already cached:

from fastapi import Request, FastAPI, Form

app = FastAPI()

@app.post("/post_name")
async def parse_data(request: Request, first_name: str = Form(...), last_name: str = Form(...)):
    data_body = await request.form()
    logging.info(data_body)  
    logging.info(first_name)
    logging.info(last_name)
    return first_name + last_name

Ref: https://www.starlette.io/requests/#body

[gor-bit]

@frankie567 This solution works if a user passes in the correct parameters e.g. first_name and last_name, along with any extra parameters not needed by the API.

However, if the user fails to pass in all of the correct parameters e.g. first_name and second_name, only a 422 response is logged. The form payload is not logged in this case.

The following curl command demonstrates this behavior with your provided code:

curl -d first_name=Sabrina -d second_name=Alami -X POST website.com/post_name

How can I capture the form payload even if I get a 422 response? This was my reasoning for using the .body method - in order to capture the entire body irrespective of the form submitted.

[frankie567]

Yes, that's how FastAPI works: if you define it as a required parameter in your endpoint, FastAPI will trigger its validation mechanism.

If you need something completely dynamic, you'll have to remove those parameters and perform the validation yourself:

from fastapi import Request, FastAPI, Form, HTTPException

app = FastAPI()

@app.post("/post_name")
async def parse_data(request: Request):
    data_body = await request.form()
    logging.info(data_body)  
    if "first_name" not in data_body or "last_name" not in data_body:
        raise HTTPException(422, "Missing fields")
    return first_name + last_name

[gor-bit]

That is very helpful, thank you @frankie567 .

For completeness, this is the final code which meets my requirements:

from fastapi import Request, FastAPI, Form, HTTPException
import logging

app = FastAPI()

@app.post("/post_name")
async def parse_data(request: Request):
    data_body = await request.form()
    logging.info(data_body)  
    if "first_name" not in data_body or "last_name" not in data_body:
        raise HTTPException(422, "Missing fields")
    first_name = data_body["first_name"]
    last_name = data_body["last_name"]
    return first_name + last_name