How to Fix FastAPI's 422 Error When POSTing JSON Data?

How to Resolve FastAPI's 422 Error When Sending JSON Data via POST Requests

The 422 Unprocessable Entity error typically occurs when a request's payload is syntactically correct, but it doesn't match the server's expectations. In this specific case, you're encountering this error because your request is trying to send JSON data to an endpoint that anticipates receiving data as query parameters.

To resolve this issue, there are multiple approaches available:

Option 1: Utilize Pydantic Models

• Pydantic models enable you to specify the expected data structure for the endpoint. The code snippet below illustrates how to define an endpoint that accepts JSON data represented as a Pydantic model:

from pydantic import BaseModel

class User(BaseModel):
    user: str

@app.post('/')
def main(user: User):
    return user

Option 2: Employ Body Parameters

• If Pydantic models are not desired, you can leverage Body parameters. The 'Body' parameter embed allows you to embed the request body as a part of the function signature:

from fastapi import Body

@app.post('/')
def main(user: str = Body(..., embed=True)):
    return {'user': user}

Option 3: Utilize Dict Type

• Another method, though less recommended, is to employ a Dict type to define key-value pairs. However, this technique doesn't support custom validations:

from typing import Dict, Any

@app.post('/')
def main(payload: Dict[Any, Any]):
    return payload

Option 4: Directly Access Request Body

• Starlette's Request object allows direct access to the parsed JSON request body using await request.json(). However, this approach doesn't offer custom validations and requires the use of async def for the endpoint definition:

from fastapi import Request

@app.post('/')
async def main(request: Request):
    return await request.json()

Testing the Options

Using Python 'requests' Library:

import requests

url = 'http://127.0.0.1:8000/'
payload = {'user': 'foo'}
resp = requests.post(url=url, json=payload)
print(resp.json())

Using JavaScript's Fetch API:

fetch('/', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json'
    },
    body: JSON.stringify({'user': 'foo'})
}).then(resp => resp.json())
    .then(data => {
        console.log(data);
    })
    .catch(error => {
        console.error(error);
    });

By selecting and implementing one of these approaches, you can successfully handle JSON data in your FastAPI endpoint, resolving the 422 error.

The above is the detailed content of How to Fix FastAPI's 422 Error When POSTing JSON Data?. For more information, please follow other related articles on the PHP Chinese website!