Custom validation with validator decorator in FastAPI - Step-by-Step Execution

Concept Flow - Custom validation with validator decorator

Define Pydantic Model

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Add @validator Decorator

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Write Validation Function

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Create Model Instance

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Trigger Validation

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Accept

This flow shows how a Pydantic model uses a @validator decorator to check data when creating an instance, accepting valid data or raising errors for invalid.

Execution Sample

FastAPI

from pydantic import BaseModel, validator

class User(BaseModel):
    name: str

    @validator('name')
    def name_must_not_be_empty(cls, v):
        if not v.strip():
            raise ValueError('Name cannot be empty')
        return v

Defines a User model with a custom validator to ensure the name is not empty or just spaces.

Execution Table

Step	Action	Input Value	Validation Check	Result
1	Create User with name='Alice'	'Alice'	Check if name is empty	Pass, name='Alice' accepted
2	Create User with name=' '	' '	Check if name is empty	Fail, raises ValueError 'Name cannot be empty'
3	Create User with name='Bob'	'Bob'	Check if name is empty	Pass, name='Bob' accepted
4	Create User with name=''	''	Check if name is empty	Fail, raises ValueError 'Name cannot be empty'

💡 Validation stops on first failure; valid names pass through.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
name	undefined	'Alice'	' '	'Bob'	''

Key Moments - 2 Insights

Why does the validator raise an error when the name is only spaces?

Because the validator uses v.strip() to remove spaces and checks if the result is empty. In execution_table row 2, the input ' ' becomes empty after strip, triggering the error.

What happens if the validator returns the value without raising an error?

The value passes validation and is accepted as shown in execution_table rows 1 and 3 where valid names are returned.

Visual Quiz - 3 Questions

Look at the execution table, what is the result when creating a User with name=' '?

A Pass, name accepted

B Pass, but name is trimmed

C Fail, raises ValueError 'Name cannot be empty'

D No validation occurs

💡 Hint

See execution_table row 2 where input ' ' fails validation.

At which step does the validation pass successfully for the name 'Bob'?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check execution_table row 3 for name='Bob' passing validation.

If the validator did not strip spaces, what would happen for input ' '?

A It would pass validation

B It would raise an error

C It would trim spaces automatically

D It would convert to None

💡 Hint

Consider how v.strip() affects validation in the code and execution_table row 2.

Concept Snapshot

Use @validator decorator in Pydantic models to add custom checks.
Define a method with cls and value parameters.
Raise ValueError to reject invalid data.
Return the value if valid.
Validation runs when creating model instances.

Full Transcript

This example shows how to add custom validation to a Pydantic model using the @validator decorator. The User model has a name field. The validator method name_must_not_be_empty checks if the name is empty or only spaces by stripping whitespace. If empty, it raises a ValueError with a message. When creating User instances, the validator runs automatically. Valid names like 'Alice' and 'Bob' pass and are accepted. Names that are empty or just spaces cause a validation error. This helps ensure data quality before using the model.