Why does a ValidationError 422 occur when sending a POST request to a FastAPI app through Postman?

Question

I cannot seem to send a POST request to a FastAPI app through Postman.

• FastAPI version 0.89.1
• Python version 3.10.9

from fastapi import FastAPI
from fastapi.params import Body
from pydantic import BaseModel

app = FastAPI()

class Post(BaseModel):
    title : str
    content : str

@app.get("/")
async def root():
    return {"message": "Hello."}

@app.post("/createposts/")
def create_posts(new_post:Post):
    print(new_post.title)
    return  {"new_post":f"title:"[new_post.title]}

I got the following error

INFO:     Finished server process [44982]
INFO:     Started server process [45121]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:64722 - "POST /createposts/ HTTP/1.1" 422 Unprocessable Entity

I'm following a tutorial and I cannot seem to find answers from other users.

I tried using the dict: Body(...) input argument instead.

I am also using Postman and this is the error:

{
    "detail": [
        {
            "loc": [
                "body"
            ],
            "msg": "value is not a valid dict",
            "type": "type_error.dict"
        }
    ]
}

Here's a screenshot of my request on Postman.

I made a POST request to the URL with the POST endpoint:

{
 "title":"a",
 "content":"b"
}

Answer 1

In Postman, you need to set the Body to be of type JSON.

In your screenshot, you have it set to Text:

It should be set to JSON:

And it should work as expected^{Note 1}, as shown in the screenshot.

As MatsLindh said in the comments, you can open the panel for the Code Snippet section to check how exactly Postman converts your request:

It should show

--header 'Content-Type: application/json' 

for JSON. You probably have it as

--header 'Content-Type: text/plain'

for text, which FastAPI cannot parse properly.

---

^{Note 1}
It's unrelated to the validation error, but you have a typo in the f-string on your endpoint. The closing double-quote on the value is misplaced.

This:

return  {"new_post":f"title:"[new_post.title]}

should be:

return  {"new_post": f"title:[{new_post.title}]"}