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.

  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.

  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).