code: def all_default_fields():
      
      default_fields = []
      for item in dir(fields):
          if not item.startswith("__"):
              var = getattr(definitions, item)
              if isinstance(var, dict):
                  if var.get('replace_null', False):
                      default_fields.append(var)
      return default_fields
docs: Helper to retrieve all fields which has default value.
  
      :returns: List of default fields.
      :rtype: list