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Sarj Sarj Sarj Standards

require-precise-factory-signature

Keep fixed keyword factory calls behind a precise callable signature.

SARJ473 Rule source Tests Catalog JSON

Why

Callable[..., T] erases argument checking, so misspelled keywords and incompatible injected factories pass strict typing and can fail at runtime.

Fix

If factory substitution is unnecessary, call the existing constructor directly. Otherwise reuse an existing precise callable contract; use type[T] when the dependency requires a class, accounting for subclass constructor compatibility. Introduce a callable Protocol only when a substitutable keyword factory actually needs a new contract; preserve the accepted keyword names instead of replacing them with unchecked **kwargs.

Examples

Before — flagged Keyword arguments escape checking
app/service.py
from collections.abc import Callable




class Result:
    pass




def build(factory: Callable[..., Result]):
    return factory(size=4)
After — preferred Factory declares its keyword contract
app/service.py
from typing import Protocol




class Result:
    pass




class Factory(Protocol):
    def __call__(self, *, size: int) -> Result: ...




def build(factory: Factory):
    return factory(size=4)