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Sarj Sarj Sarj Standards

no-before-validator-input-mutation

Preserve raw inputs in Pydantic before validators.

SARJ482 Rule source Tests Catalog JSON

Why

Treat raw before-validator inputs as borrowed data. They can be caller-owned or reused by another validation branch; mutation can leak normalization into aliases and leave data changed after validation fails. Return replacements even when a particular caller supplies fresh data.

Fix

Return a normalized replacement mapping or collection. Copy the changed path before updating nested values; a fresh outer copy permits outer writes but retains shared descendants.

Examples

Before — flagged Removing a raw entry changes caller data
app/records.py
from pydantic import BaseModel, model_validator




class Record(BaseModel):
    name: str


    @model_validator(mode="before")
    @classmethod
    def normalize(cls, data: dict[str, object]):
        data.pop("legacy", None)
        return data
After — preferred A copied outer mapping can be updated locally
app/records.py
from pydantic import BaseModel, model_validator




class Record(BaseModel):
    name: str


    @model_validator(mode="before")
    @classmethod
    def normalize(cls, data: dict[str, object]):
        data = dict(data)
        data["name"] = "new"
        return data
Before — flagged Before-validation changes the original mapping
app/records.py
from pydantic import BaseModel, model_validator




class Record(BaseModel):
    name: str


    @model_validator(mode="before")
    @classmethod
    def normalize(cls, data: dict[str, object]):
        data["name"] = "new"
        return data
After — preferred Return normalized data without changing the original
app/records.py
from pydantic import BaseModel, model_validator




class Record(BaseModel):
    name: str


    @model_validator(mode="before")
    @classmethod
    def normalize(cls, data: dict[str, object]):
        return {**data, "name": "new"}