Custom validation with validator decorator in FastAPI

Introduction

Custom validation lets you check data rules beyond basic types. It helps keep your app data clean and correct.

When you want to check if a string matches a special pattern like a phone number.

When you need to ensure a number is within a certain range.

When you want to validate one field based on another field's value.

When built-in validations are not enough for your data rules.

Syntax

FastAPI

from pydantic import BaseModel, validator

class ModelName(BaseModel):
    field_name: field_type

    @validator('field_name')
    def check_field_name(cls, value):
        # validation logic
        valid = True  # replace with actual validation condition
        if not valid:
            raise ValueError('error message')
        return value

The @validator decorator is used inside a Pydantic model class.

The validator method must return the value if it passes validation.

Examples

This checks that the username is not just spaces or empty.

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    username: str

    @validator('username')
    def username_must_not_be_empty(cls, v):
        if not v.strip():
            raise ValueError('Username cannot be empty')
        return v

This ensures the price is greater than zero.

FastAPI

from pydantic import BaseModel, validator

class Product(BaseModel):
    price: float

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

Sample Program

This FastAPI app defines an Item model with a custom validator that checks quantity is positive. If you send a POST request with quantity 0 or less, it returns an error. Otherwise, it returns the item data.

FastAPI

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

app = FastAPI()

class Item(BaseModel):
    name: str
    quantity: int

    @validator('quantity')
    def quantity_must_be_positive(cls, v):
        if v <= 0:
            raise ValueError('Quantity must be greater than zero')
        return v

@app.post('/items/')
async def create_item(item: Item):
    return JSONResponse(content={'name': item.name, 'quantity': item.quantity})

Important Notes

Validators run automatically when you create or update model instances.

You can add multiple validators for different fields in the same model.

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

Summary

Use @validator inside Pydantic models to add custom checks.

Validators help keep your data clean and your app safe.

Always return the value if it passes validation or raise an error if not.

Practice

1. What is the main purpose of using the @validator decorator in a Pydantic model within FastAPI?

easy

A. To define database relationships

B. To automatically generate API routes

C. To add custom validation logic for model fields

D. To format the output JSON response

Solution

1. Step 1: Understand the role of @validator

   The @validator decorator is used inside Pydantic models to add custom validation logic for fields.

2. Step 2: Differentiate from other functionalities

   It does not generate routes, define database relations, or format output JSON; those are handled elsewhere.

3. Final Answer:

   To add custom validation logic for model fields -> Option C

4. Quick Check:

   Custom validation = To add custom validation logic for model fields [OK]

Hint: Remember: @validator checks field values before use [OK]

Common Mistakes:

• Confusing validation with route creation
• Thinking it formats output JSON
• Assuming it manages database relations

2. Which of the following is the correct syntax to create a validator for the field email in a Pydantic model?

easy

A. @validator['email']\ndef check_email(cls, value):\n return value

B. @validate('email')\ndef check_email(cls, value):\n return value

C. @validator(email)\ndef check_email(cls, value):\n return value

D. @validator('email')\ndef check_email(cls, value):\n return value

Solution

1. Step 1: Identify correct decorator usage

   The correct decorator is @validator('field_name') with the field name as a string.

2. Step 2: Check syntax correctness

   @validator('email')\ndef check_email(cls, value):\n return value uses @validator('email') correctly; others use wrong decorator names or syntax.

3. Final Answer:

   @validator('email')\ndef check_email(cls, value):\n return value -> Option D

4. Quick Check:

   Correct decorator syntax = @validator('email')\ndef check_email(cls, value):\n return value [OK]

Hint: Use quotes around field name in @validator [OK]

Common Mistakes:

• Omitting quotes around field name
• Using wrong decorator name like @validate
• Using brackets [] instead of parentheses ()

3. Given the following Pydantic model, what will happen if you create an instance with age=15?

from pydantic import BaseModel, validator

class User(BaseModel):
    age: int

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

user = User(age=15)

medium

A. A ValueError is raised with message 'Must be at least 18'

B. The user instance is created with age 15

C. The age is automatically set to 18

D. A TypeError is raised because age is int

Solution

1. Step 1: Analyze the validator logic

   The validator checks if age is less than 18 and raises a ValueError if true.

2. Step 2: Apply input value to validator

   Since age=15 is less than 18, the validator raises the error with the message.

3. Final Answer:

   A ValueError is raised with message 'Must be at least 18' -> Option A

4. Quick Check:

   Age < 18 triggers ValueError = A ValueError is raised with message 'Must be at least 18' [OK]

Hint: Validator raises error if condition fails [OK]

Common Mistakes:

• Assuming instance is created despite validation error
• Thinking age auto-corrects to 18
• Confusing ValueError with TypeError

4. Identify the error in this validator code snippet:

from pydantic import BaseModel, validator

class Product(BaseModel):
    price: float

    @validator('price')
    def check_price(value):
        if value <= 0:
            raise ValueError('Price must be positive')
        return value

medium

A. Validator decorator should be @validate

B. Missing cls parameter in validator method

C. Return statement is missing

D. Field name in decorator should be price without quotes

Solution

1. Step 1: Check validator method signature

   Validator methods must have cls as the first parameter, then the value.

2. Step 2: Identify missing parameter

   The method only has value, missing cls, causing a TypeError.

3. Final Answer:

   Missing cls parameter in validator method -> Option B

4. Quick Check:

   Validator method needs cls first = Missing cls parameter in validator method [OK]

Hint: Validator methods always start with cls, then value [OK]

Common Mistakes:

• Omitting cls parameter
• Using wrong decorator name
• Forgetting to return the value

5. You want to create a Pydantic model that validates a username field to ensure it is at least 3 characters long and contains only letters and numbers. Which validator implementation correctly achieves this?

hard

A. from pydantic import BaseModel, validator
import re

class User(BaseModel):
username: str

@validator('username')
def valid_username(cls, v):
if len(v) < 3 or not re.match(r'^[a-zA-Z0-9]+$', v):
raise ValueError('Username must be 3+ chars and alphanumeric')
return v

B. from pydantic import BaseModel, validator

class User(BaseModel):
username: str

@validator('username')
def valid_username(cls, v):
if len(v) > 3 and v.isalnum():
raise ValueError('Username must be 3+ chars and alphanumeric')
return v

C. from pydantic import BaseModel, validator

class User(BaseModel):
username: str

@validator('username')
def valid_username(cls, v):
if len(v) < 3 or not v.isalpha():
raise ValueError('Username must be 3+ chars and letters only')
return v

D. from pydantic import BaseModel, validator
import re

class User(BaseModel):
username: str

@validator('username')
def valid_username(cls, v):
if len(v) < 3 or not re.match(r'^[a-z]+$', v):
raise ValueError('Username must be 3+ chars and lowercase letters only')
return v

Solution

1. Step 1: Check length and character conditions

   The username must be at least 3 characters and only letters or numbers.

2. Step 2: Evaluate each option's validation logic

   from pydantic import BaseModel, validator import re class User(BaseModel): username: str @validator('username') def valid_username(cls, v): if len(v) < 3 or not re.match(r'^[a-zA-Z0-9]+$', v): raise ValueError('Username must be 3+ chars and alphanumeric') return v correctly checks length & uses regex to allow letters and numbers only. Others either check wrong conditions or restrict letters only.

3. Final Answer:

   Option A correctly validates username length and alphanumeric characters -> Option A

4. Quick Check:

   Length >=3 and alphanumeric = from pydantic import BaseModel, validator import re class User(BaseModel): username: str @validator('username') def valid_username(cls, v): if len(v) < 3 or not re.match(r'^[a-zA-Z0-9]+$', v): raise ValueError('Username must be 3+ chars and alphanumeric') return v [OK]

Hint: Use regex to check alphanumeric and length together [OK]

Common Mistakes:

• Using isalpha() instead of isalnum()
• Checking length incorrectly (greater than vs less than)
• Restricting to letters only when numbers allowed