How to Handle FastAPI's 422 Error When Receiving JSON POST Requests?

FastAPI: Error 422 with POST Request When Sending JSON Data

When constructing RESTful APIs, it's common to encounter issues related to data exchange, particularly when POST requests are involved. One such issue is receiving a "422 Unprocessable Entity" error while attempting to send JSON data.

In the provided code example:

from fastapi import FastAPI

app = FastAPI()

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

This code defines a POST endpoint that expects a JSON payload containing a "user" key. However, the error occurs when the HTTP client sends JSON data that doesn't match the expected format. To resolve this, there are several options:

Option 1: Using Pydantic Models

Pydantic models provide a way to validate and deserialize JSON payloads according to predefined schemas:

from pydantic import BaseModel

class User(BaseModel):
    user: str

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

Option 2: Using Body Parameters

Body parameters in FastAPI allow you to directly parse the JSON payload without defining a Pydantic model:

from fastapi import Body

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

Option 3: Using Dict Type

While less recommended, you can use a Dict type to receive the JSON payload as a key-value pair:

from typing import Dict, Any

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

Option 4: Using Request Body Directly

If you're sure that the incoming data is valid JSON, you can use Starlette's Request object to parse it:

from fastapi import Request

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

Testing the Options

You can test these 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())

JavaScript Fetch API:

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

By implementing one of these approaches, you can resolve the 422 error and successfully handle JSON data in your FastAPI POST endpoints.

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