Custom validators in FastAPI

Introduction

Custom validators help check if data fits special rules you make. This keeps your app safe and correct.

You want to check if a username has no spaces.

You need to ensure a password has a mix of letters and numbers.

You want to confirm an email ends with a certain domain.

You want to make sure a date is not in the past.

You want to validate a custom ID format.

Syntax

FastAPI

from pydantic import BaseModel, validator

class MyModel(BaseModel):
    field_name: str

    @validator('field_name')
    def check_field(cls, value):
        # your custom check here
        if not value.isalpha():
            raise ValueError('Only letters allowed')
        return value

Use @validator('field_name') above a method to check that field.

The method gets the value and must return it if valid or raise ValueError if not.

Examples

This checks the username has no spaces.

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    username: str

    @validator('username')
    def no_spaces(cls, v):
        if ' ' in v:
            raise ValueError('No spaces allowed')
        return v

This ensures the price is more than zero.

FastAPI

from pydantic import BaseModel, validator

class Product(BaseModel):
    price: float

    @validator('price')
    def positive_price(cls, v):
        if v <= 0:
            raise ValueError('Price must be positive')
        return v

This forces the email to end with '@gmail.com'.

FastAPI

from pydantic import BaseModel, validator

class EmailCheck(BaseModel):
    email: str

    @validator('email')
    def must_be_gmail(cls, v):
        if not v.endswith('@gmail.com'):
            raise ValueError('Email must be gmail')
        return v

Sample Program

This FastAPI app has a User model with two custom checks: username cannot have spaces, and age must be 18 or older. If data fails, FastAPI returns an error automatically.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, validator
from fastapi.responses import JSONResponse

app = FastAPI()

class User(BaseModel):
    username: str
    age: int

    @validator('username')
    def username_no_spaces(cls, v):
        if ' ' in v:
            raise ValueError('Username cannot contain spaces')
        return v

    @validator('age')
    def age_must_be_adult(cls, v):
        if v < 18:
            raise ValueError('Must be at least 18 years old')
        return v

@app.post('/users')
async def create_user(user: User):
    return JSONResponse(content={'message': f'User {user.username} created, age {user.age}'})

Important Notes

Custom validators run automatically when data is received or created.

Raise ValueError with a clear message to help users fix input.

You can add multiple validators for different fields or the same field.

Summary

Custom validators check data with your own rules.

They help keep your app safe and user input correct.

Use @validator on model fields to add these checks.