Ensure configuration behavior is predictable, validated, and migration-friendly: - Precedence: When combining env vars, settings, and defaults, explicitly define and implement the precedence order (and document/announce any change). Avoid “surprising” priority flips. - Defaults & transitions: Choose defaults for backward compatibility, but provide a clear deprecation/transition plan for behavior changes. For feature flags, minimize error-prone user responsibility—prefer safe/automatic behavior when possible, and document operational warnings. - Config schema: Add new options to the formal defaults/conf schema and then access them directly (avoid optional `get()` patterns when the option is already defined). - Validation: Validate config values and fail fast for invalid/misconfigured settings (raise `ImproperlyConfigured` rather than using silent fallbacks). If a setting is expected to be callable/type-specific, check it. Example (fail-fast validation for a config-provided function): ```python from celery.exceptions import ImproperlyConfigured repr_function = self.app.conf.task_args_repr_function if not repr_function: raise ImproperlyConfigured("task_args_repr_function must be set") if isinstance(repr_function, str): # resolve symbol_by_name(...) repr_function = symbol_by_name(repr_function) if not callable(repr_function): raise ImproperlyConfigured("task_args_repr_function must be callable") ``` Example (explicit precedence): - Decide whether `broker_url` or `CELERY_BROKER_*_URL` wins, implement that order consistently, and ensure it’s documented as a behavior contract.