main.py

from typing import Optional

from fastapi import FastAPI
from pydantic import BaseModel, validator


class Item(BaseModel):
    name: str
    description: Optional[str] = None
    price: float
    tax: Optional[float] = None

    @validator('name')
    def name_must_contain_space(cls, v):
        if ' ' not in v:
            raise ValueError('must contain a space')
        return v.title()

    @validator('description')
    def description_length(cls, v):
        if len(v) < 10:
            raise ValueError('description must be 3 chars long')
        return v


app = FastAPI()


@app.post("/items/")
async def create_item(item: Item):
    return item