When handling optional/nullable inputs, treat “missing” and “explicitly set to None” as different semantic states. Practical rules: - Provide safe fallbacks at the boundary: if a config/input can be `None`, use a default that keeps the value non-null. ```python task_id_generator = conf.get("task_id_generator", uuid) if task_id is None: task_id = task_id_generator() ``` - Preserve explicit caller/request intent: if APIs accept `time_limit=None` (meaning “explicit”), do not collapse it with “argument omitted”. Resolve defaults only when the caller did not provide a value. - For override fields where frameworks use class-level defaults, you must detect “was set on the instance” vs “fell back to class default”. Using only `getattr(x, 'field', None)` can be misleading; use presence checks (e.g., `__dict__`) before applying fallback logic. ```python actual = getattr(req, 'ignore_result', None) if isinstance(req, Context) and 'ignore_result' not in req.__dict__: actual = None # treat as “unset” to allow task-level fallback ``` - Early-exit on truly-null dependencies to prevent latent exceptions: `if producer is None: return`. - Avoid mutable defaults for optional dict-like arguments (use `None` and initialize inside the function).