validation and nested model

[foulong]

Description

How can I use validation on fields of model ?

I wrote my code with this inspiration : discussion on #312
It runs however, i want to handle error, but where do i push my try/except ?

from fastapi import FastAPI
from pydantic import BaseModel, ValidationError, validator, Schema
from typing import List


app = FastAPI()


class Item(BaseModel):
    v1: float = Schema(0, gte=-90, lte=90)
    v2: float

    @validator('v1' , 'v2')
    def check_value(cls, v):
        if v < 0 or v > 10:
            raise ValueError(f'error value')
        return v


class ListItem(BaseModel):
    data: List[Item] = None

@app.post("/items/")
async def create_item(item: ListItem):   
        nb_items = len(item.data)
        the_sum = 0
        i = 0
    
        while (i < nb_items):
            l1 = item.data[i]
            the_sum += l1.v1 * 1.5 / l1.v2;
            i += 1

    return {"result": the_sum }

Thanks in advance for help.

[phy25]

Well, since you define it at the function signature level, usually FastAPI will help you validate against Pydantic validation rules and raise HTTP error appropriately before executing your router function, so you cannot try...catch... in your router function.

If you need it to be more flexible, you can either

• do router-based validation inside your router function rather than do it as a Pydantic validator, or,
• handle error yourself by using a global exception handler: https://fastapi.tiangolo.com/tutorial/handling-errors/#override-request-validation-exceptions

[foulong]

Hi @phy25

thanks for your help.

I thought that router-based validation would cause many work to have an equivalent of a pydantic validator.
What do you think ?

I worked from your second suggestion, handle error myself using a global exception handler.

...

from fastapi.exceptions import RequestValidationError
from fastapi.responses import PlainTextResponse
import re

class Item(BaseModel):
    v1: float = Schema(0, gte=-90, lte=90)
    v2: float

    @validator('v1' , 'v2')
    def check_value(cls, v, field):
        if v < 0 or v > 10:
            raise ValueError(str(field.name) + ": doit être entre [0; 10]")

        return v


@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request, exc):
    errors = str(exc).splitlines()
    justErrorText = re.compile(r'(.+)\s+(\(.+\))').sub(r"\1' ", errors[2])
    
    return PlainTextResponse(justErrorText, status_code=400)

I wanted retrieve the text of valueError. That's why, i must remove informations from the second parameter of validation_exception_handler.

What do you think ?

maybe i could do better, and i will be delighted to have your opinion.

[tiangolo]

Thanks for the help @phy25 ! 🍰 🙇

@foulong it's in the docs, check it here: https://fastapi.tiangolo.com/tutorial/handling-errors/#use-the-requestvalidationerror-body

[github-actions[bot]]

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