Why is there an explicit cast to list for filter backends?

[sevdog]

In the generics package there is a explicit cast to list of filter_backends:

django-rest-framework/rest_framework/generics.py

Line 153 in d3dd45b

for backend in list(self.filter_backends):

In the docs it states that that attribute should be a list:

filter_backends - A list of filter backend classes that should be used for filtering the queryset. Defaults to the same value as the DEFAULT_FILTER_BACKENDS setting.

And in the unittests that attribute is always a list or a tuple.

Why it was left that "casting" which results in a copy of the attribute while other similar attributes (renderer_classes, parser_classes, authentication_classes, permission_classes and throttle_classes) does not pass through a copy/casting mechanism?

How many time is wasted by this operation in the current DRF?