https://github.com/tiangolo/fastapi/issues/2149

none_fields_example.py

from fastapi import FastAPI

app = FastAPI()

from typing import Optional
from uvicorn import run

from pydantic import BaseModel, Field


class AddressResponse1(BaseModel):
    city: Optional[str] = Field(..., nullable=True, example="London")
    street: Optional[str] = Field(..., nullable=True, example="Baker street")
    house: Optional[str] = Field(..., nullable=True, example="221")
    block: Optional[str] = Field(..., nullable=True, example=None)


@app.get('v1/address', response_model=AddressResponse1)
async def get_address_v1():
    return AddressResponse1(city='London', street='Baker street', house='221', block=None)


class AddressResponse2(BaseModel):
    city: Optional[str] = Field(..., nullable=True)
    street: Optional[str] = Field(..., nullable=True)
    house: Optional[str] = Field(..., nullable=True)
    block: Optional[str] = Field(..., nullable=True)

    class Config:
        schema_extra = {
            "example": {
                "city": "London",
                "street": "Baker street",
                "house": "221",
                "block": None
            }
        }


@app.get('v2/address', response_model=AddressResponse2)
async def get_address_v2():
    return AddressResponse2(city='London', street='Baker street', house='221', block=None)


run(app, debug=True)