FastAPI POST Request Error 422: How to Properly Handle JSON Data?

FastAPI: Understanding and Resolving Error 422 when Sending JSON Data via POST Request

Problem Description

Building an API with FastAPI, the application exhibits error code 422 (Unprocessable Entity) while sending JSON data through a POST request. GET requests, on the other hand, work smoothly. This issue persists despite attempts to parse JSON, use UTF-8 encoding, or modify HTTP headers.

Detailed Explanation

A response with the 422 status code indicates an error with the submitted data, often due to missing or incorrectly formatted elements. In this case, the problem stems from the expected format of the POST request. By default, FastAPI anticipates user input as a query parameter, not as JSON payload. Consequently, the client's attempt to transmit JSON data results in the 422 error.

Solution Options

The following four options offer distinct approaches to defining an endpoint that correctly processes JSON data from a POST request:

1. Using Pydantic Models

Employing Pydantic models enables you to specify an expected data structure for your endpoint.

from pydantic import BaseModel

class User(BaseModel):
    user: str

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

2. Using the Body Parameter Embed

This method utilizes the special "embed" parameter to treat the body as a single parameter.

from fastapi import Body

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

3. Using a Dict Type (Less Recommended)

While less recommended, this approach defines a key-value pair as a Dict type.

from typing import Dict, Any

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

4. Directly Using the Request Object

This option involves using the Request object to parse the received JSON data.

from fastapi import Request

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

Testing the Solutions

To test the provided solutions, follow the given steps:

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 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);
    });

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