Custom validation with validator decorator in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when invalid data is passed to this Pydantic model?

Consider this FastAPI Pydantic model using a @validator decorator:

from pydantic import BaseModel, validator

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

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

user = User(username='alice', age=16)

What will happen when this code runs?

FastAPI

from pydantic import BaseModel, validator

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

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

user = User(username='alice', age=16)

A It raises a ValueError with message 'Must be at least 18' during model creation.

B It creates the User object with age 16 without error.

C It raises a TypeError because age is not a string.

D It ignores the validator and sets age to 16.

💡 Hint

Think about what the validator function does when the age is less than 18.

The @validator decorator runs the age_must_be_adult method when the model is created. Since age is 16, which is less than 18, the method raises a ValueError. This stops the model from being created.

📝 Syntax

intermediate

Which option correctly uses the validator decorator to check multiple fields?

You want to validate that start_date is before end_date in a Pydantic model. Which code snippet correctly implements this using @validator?

FastAPI

from pydantic import BaseModel, validator
from datetime import date

class Event(BaseModel):
    start_date: date
    end_date: date

    # Your validator here

A

    @validator('end_date')
    def check_dates(cls, v, values):
        if 'start_date' in values and v <= values['start_date']:
            raise ValueError('end_date must be after start_date')
        return v

B

    @validator('start_date', 'end_date')
    def check_dates(cls, v):
        if v['start_date'] >= v['end_date']:
            raise ValueError('start_date must be before end_date')
        return v

C

    @validator('start_date')
    def check_dates(cls, v, values):
        if v >= values['end_date']:
            raise ValueError('start_date must be before end_date')
        return v

D

    @validator('start_date', 'end_date')
    def check_dates(cls, start, end):
        if start >= end:
            raise ValueError('start_date must be before end_date')
        return start

💡 Hint

Remember the validator receives the current field value and a dict of previously validated fields.

Option A correctly validates end_date by checking if start_date is already validated in values. It raises an error if end_date is not after start_date. Other options misuse parameters or try to access fields incorrectly.

🔧 Debug

advanced

Why does this validator not run as expected?

Look at this Pydantic model:

from pydantic import BaseModel, validator

class Product(BaseModel):
    name: str
    price: float

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

product = Product(name='Book', price=0)

Why does this code raise no error even though price is 0?

FastAPI

from pydantic import BaseModel, validator

class Product(BaseModel):
    name: str
    price: float

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

product = Product(name='Book', price=0)

A The validator decorator is missing parentheses, so it is not applied.

B The error is raised but caught internally by Pydantic, so no exception shows.

C The price field is not declared as Optional, so validation is ignored.

D The validator function does not return the value, so validation is skipped silently.

💡 Hint

Check what the validator function returns.

Validators must return the value after validation. Since price_positive does not return anything, it returns None implicitly, which Pydantic treats as valid. So the validation does not work as intended.

❓ state_output

advanced

What is the value of 'email' after model creation?

Given this Pydantic model:

from pydantic import BaseModel, validator

class User(BaseModel):
    email: str

    @validator('email')
    def normalize_email(cls, v):
        return v.lower().strip()

user = User(email='  [e-mail]  ')

What is the value of user.email?

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    email: str

    @validator('email')
    def normalize_email(cls, v):
        return v.lower().strip()

user = User(email='  [e-mail]  ')
result = user.email

A Raises a ValueError because of whitespace

B ' [e-mail] '

C '[e-mail]'

D '[e-mail]'

💡 Hint

Think about what lower() and strip() do to the string.

The validator converts the email to lowercase and removes spaces at the start and end. So the stored email is '[e-mail]'.

🧠 Conceptual

expert

Which statement about Pydantic's @validator decorator is TRUE?

Choose the correct statement about how the @validator decorator works in Pydantic models.

A Validators must always be instance methods and cannot be class methods.

B Validators can access other fields' values only if those fields are validated before the current field.

C Validators run after the entire model is created and can modify any field arbitrarily.

D Validators automatically run asynchronously if the decorated method is async.

💡 Hint

Consider the order of field validation and how values parameter works.

Validators run in the order fields are declared. They receive a values dict with already validated fields. So they can only access fields validated before them. Validators are class methods, not instance methods. Async validators are not supported automatically.