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Sarj Sarj Sarj Standards

no-input-model-mutation

Return replacement values instead of mutating caller-owned Pydantic models.

SARJ481 Rule source Tests Catalog JSON

Why

Assignments through shared model inputs also change every caller alias. Copying only the outer list or model retains nested aliases and can make repeated transformations observe each other's changes.

Fix

Return replacement models and explicitly capture the result. Copy each changed ancestor; permit fresh local builders and use exact suppressions for documented in-place APIs. model_copy updates must already satisfy the complete model contract.

Examples

Before — flagged A shallow model copy shares a nested model
app/records.py
from pydantic import BaseModel




class Attachment(BaseModel):
    url: str




class Record(BaseModel):
    attachment: Attachment




def sign(record: Record):
    copied = record.model_copy()
    copied.attachment.url = "signed"
After — preferred Replace the nested value and its parent
app/records.py
from pydantic import BaseModel




class Attachment(BaseModel):
    url: str




class Record(BaseModel):
    attachment: Attachment




def sign(record: Record):
    attachment = record.attachment.model_copy(update={"url": "signed"})
    return record.model_copy(update={"attachment": attachment})
Before — flagged A transformation changes its caller's model
app/records.py
from pydantic import BaseModel




class Record(BaseModel):
    name: str




def rename(record: Record):
    record.name = "new"
After — preferred Return a trusted replacement
app/records.py
from pydantic import BaseModel




class Record(BaseModel):
    name: str




def rename(record: Record):
    return record.model_copy(update={"name": "new"})