Custom validation with validator decorator in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @validator decorator in FastAPI's Pydantic models?

The @validator decorator is used to add custom validation logic to model fields. It lets you check or modify field values before the model is created.

intermediate

How do you apply a validator to multiple fields using the @validator decorator?

You pass a list of field names to the @validator decorator. The decorated method will run for each of those fields.

beginner

What should a validator method return in FastAPI's Pydantic model?

A validator method must return the validated (and possibly modified) value for the field. Returning the wrong type or nothing will cause errors.

intermediate

Can you explain the difference between @validator and @root_validator in FastAPI?

@validator validates individual fields, while @root_validator validates the whole model at once, allowing checks involving multiple fields.

beginner

What happens if a validator raises a ValueError in FastAPI?

Raising a ValueError inside a validator stops model creation and returns a clear error message to the user, explaining what went wrong.

What does the @validator decorator in FastAPI do?

A Adds custom validation logic to model fields

B Creates database tables automatically

C Handles HTTP requests

D Generates API documentation

The @validator decorator is specifically for adding custom validation to fields in Pydantic models used by FastAPI.

How do you specify which fields a validator should apply to?

A By setting a global config

B By naming the validator method after the field

C By passing field names as arguments to <code>@validator</code>

D By using <code>@root_validator</code>

You list the field names inside the @validator decorator to apply the validator to those fields.

What must a validator method return?

A Nothing

B The validated or modified field value

C A boolean true or false

D A new model instance

Validator methods must return the value that will be set on the field after validation.

Which validator type checks multiple fields together?

A @root_validator

B @validator

C @field_validator

D @model_validator

@root_validator runs after all fields are validated and can check relationships between fields.

What happens if a validator raises a ValueError?

A The server crashes

B The error is ignored

C The field is set to None

D Model creation fails with an error message

Raising ValueError stops model creation and returns a clear validation error to the client.

Explain how to create a custom validator for a Pydantic model field using the @validator decorator in FastAPI.

Think about how you check and fix data before saving it.

Describe the difference between @validator and @root_validator in FastAPI's Pydantic models.

One works on one field, the other on the whole form.