Request bodies: POST with a Pydantic model

Parameters suit an id or an option. A new ticket is a whole record, so it travels in the request body as JSON, and a Pydantic model says what that JSON must contain.

Exampletickets.py

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()
TICKETS = []


class NewTicket(BaseModel):
    customer: str
    text: str


@app.post("/tickets")
def create_ticket(ticket: NewTicket):
    record = {"id": len(TICKETS) + 1, **ticket.model_dump()}
    TICKETS.append(record)
    return record

A parameter whose type is a Pydantic model is read from the body. FastAPI reads the JSON, builds a NewTicket from it, and only then calls your function. TICKETS is a list in memory, so it starts empty each time the app starts.

Example

from fastapi.testclient import TestClient
from tickets import app

client = TestClient(app)
response = client.post("/tickets", json={"customer": "Asha", "text": "I was charged twice"})
print(response.status_code)
print(response.json())

A body missing a field

Example

response = client.post("/tickets", json={"customer": "Ben"})
print(response.status_code)
print(response.json())

422 again, this time with loc pointing into the body at text, and input showing what was actually sent. The client learns exactly what to fix, and your function never has to check.

Extra fields

Example

response = client.post("/tickets", json={"customer": "Chen", "text": "Refund please", "admin": True})
print(response.json())

admin is not in the model, so it is dropped silently. A client cannot slip a field into your record by sending it.

Try it yourself

Add priority: int = 3 to NewTicket and post a ticket with and without it.
Post {"customer": 5, "text": "hi"}. Is a number accepted as a string?
Post two tickets and check the second one's id.

PreviousPath and query parameters

Little by little, you're building something great.