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Sarj Sarj Sarj Standards

no-nested-pydantic-field-validator

Outer-model Pydantic field validator is owned by a nested helper class.

SARJ425 Rule source Tests Catalog JSON

Why

Pydantic collects validator metadata from the model class namespace. A decorator indented into a nested class is therefore invisible to the outer model, so its declared field silently loses validation.

Fix

Move the field-validator method into the outer model class.

Examples

Before — flagged Nested Config silently does not validate Settings.language
app/models.py
from pydantic import BaseModel, field_validator




class Settings(BaseModel):
    language: str


    class Config:
        extra = "forbid"


        @field_validator("language")
        @classmethod
        def normalize(cls, value):
            return value.lower()
After — preferred Validator belongs to the model
app/models.py
from pydantic import BaseModel, field_validator




class Settings(BaseModel):
    language: str


    @field_validator("language")
    @classmethod
    def normalize(cls, value):
        return value.lower()