Create reference slice to a list in Python

list_reference_copy.py

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

class MyList(list):
    def __init__(self, raw_list, start=None, stop=None):
        self.raw_list = raw_list
        self.start = start or 0
        self.stop = stop or len(raw_list)-1

    def __getitem__(self, slice_):
        return MyList(self.raw_list, start=self.start+slice_.start,
                      stop=self.start+slice_.stop)

    def __setitem__(self, slice_, value):
        self.raw_list[slice_] = value

    def __str__(self):
        return str(self.raw_list[self.start:self.stop])

def foo(l):
    l[:] = [999]

if __name__ == '__main__':
    l = [1, 2, 3, 4]
    ref_copy = MyList(l)
    ref_copy[0] = 999
    assert l == [999, 2, 3, 4]

    foo(l)
    assert l == [999]

    l[:] = [1, 2, 3, 4, 5, 6]

    assert str(ref_copy[1:3]) == str([2, 3])
    assert str(ref_copy[1:3][1:2]) == str([3])