pyhton's collections.DefaultDict but for lists. Quick mockup

DefaultList.py

class DefaultList(list):
    def __init__(self, list_, default=None):
        super().__init__(list_)
        self.default = default
    
    def __getitem__(self, index):
        if isinstance(index, slice):            
            start = index.start
            stop = index.stop
            step = index.step
            length = super().__len__()
            
            # [1,2,3][:]
            if stop is None:
                return super().__getitem__(index)
            
            # [1,2,3][:100]
            if length < stop:
                extra = [self.default] * (stop - length - 1)
                original = super().__getitem__(slice(start, length))
                return (original + extra)[slice(None, None, step)]
        try:
            # [1,2,3][0]
            return super().__getitem__(index)
        except IndexError:
            # [1,2,3][100]
            return self.default

def test_DefaultList():
    li = [0,1,2,3,4,5,6,7,8]
    dli = DefaultList([0,1,2,3,4,5,6,7,8])
    
    # listlike
    assert dli[:] == li[:]
    assert dli[:3] == li[:3]
    assert dli[-3:] == li[-3:]
    assert dli[::2] == li[::2]
    assert dli[3:-3] == li[3:-3]
    
    # non-listlike
    assert dli[10] == None
    assert dli[10:12] == [None, None]
    assert dli[7:11] == [7,8,None]
    assert dli[-2:11] == [7,8,None]
    
    # methods
    dli.append(9)
    li.append(9)
    
    dli == li
    
    dli.pop() == li.pop()
    
test_DefaultList()