Make every field as optional with Pydantic

Question

I'm making an API with FastAPI and Pydantic.

I would like to have some PATCH endpoints, where 1 or N fields of a record could be edited at once. Moreover, I would like the client to only pass the necessary fields in the payload.

Example:

class Item(BaseModel):
    name: str
    description: str
    price: float
    tax: float


@app.post("/items", response_model=Item)
async def post_item(item: Item):
    ...

@app.patch("/items/{item_id}", response_model=Item)
async def update_item(item_id: str, item: Item):
    ...

In this example, for the POST request, I want every field to be required. However, in the PATCH endpoint, I don't mind if the payload only contains, for example, the description field. That's why I wish to have all fields as optional.

Naive approach:

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

But that would be terrible in terms of code repetition.

Any better option?

Answer 1

This method prevents data validation

Read this by @Anime Bk: https://stackoverflow.com/a/75011200

Solution with metaclasses

I've just come up with the following:

class AllOptional(pydantic.main.ModelMetaclass):
    def __new__(cls, name, bases, namespaces, **kwargs):
        annotations = namespaces.get('__annotations__', {})
        for base in bases:
            annotations.update(base.__annotations__)
        for field in annotations:
            if not field.startswith('__'):
                annotations[field] = Optional[annotations[field]]
        namespaces['__annotations__'] = annotations
        return super().__new__(cls, name, bases, namespaces, **kwargs)

Use it as:

class UpdatedItem(Item, metaclass=AllOptional):
    pass

So basically it replace all non optional fields with Optional

Any edits are welcome!

With your example:

from typing import Optional

from fastapi import FastAPI
from pydantic import BaseModel
import pydantic

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str
    price: float
    tax: float


class AllOptional(pydantic.main.ModelMetaclass):
    def __new__(self, name, bases, namespaces, **kwargs):
        annotations = namespaces.get('__annotations__', {})
        for base in bases:
            annotations.update(base.__annotations__)
        for field in annotations:
            if not field.startswith('__'):
                annotations[field] = Optional[annotations[field]]
        namespaces['__annotations__'] = annotations
        return super().__new__(self, name, bases, namespaces, **kwargs)

class UpdatedItem(Item, metaclass=AllOptional):
    pass

# This continues to work correctly
@app.get("/items/{item_id}", response_model=Item)
async def get_item(item_id: int):
    return {
        'name': 'Uzbek Palov',
        'description': 'Palov is my traditional meal',
        'price': 15.0,
        'tax': 0.5,
    }

@app.patch("/items/{item_id}") # does using response_model=UpdatedItem makes mypy sad? idk, i did not check
async def update_item(item_id: str, item: UpdatedItem):
    return item