Python: FastAPI error 422 with POST request when sending JSON data

General
1

I’m building a simple API to test a database. When I use GET request everything works fine, but if I change to POST, I get 422 Unprocessable Entity error.

Here is the FastAPI code:

from fastapi import FastAPI

app = FastAPI()

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

Here is my JavaScript request:

let axios = require('axios')

data = { 
    user: 'smith' 
}

axios.post('http://localhost:8000', data)
    .then(response => (console.log(response.url)))

Also, using Python requests:

import requests

url = 'http://127.0.0.1:8000'
data = {'user': 'Smith'}

response = requests.post(url, json=data)
print(response.text)

I also tried to parse as JSON, enconding using utf-8, and change the headers, but nothing worked for me.

2

Your FastAPI POST endpoint is missing a request body model or a way to tell FastAPI to expect JSON input. That’s why you get 422 Unprocessable Entity — FastAPI doesn’t know how to parse the user parameter.

How to fix it:

Use a Pydantic model to define the expected request body, like this:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class UserRequest(BaseModel):
    user: str

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

Now FastAPI knows to expect JSON with a user field and will parse it correctly.

Your JavaScript and Python requests sending JSON are fine.

Summary:

  • Define a Pydantic model for your POST body.
  • Use that model as the parameter in your endpoint.
  • Then your POST requests will work without 422 errors.