Python/sage: can lists start at index 1?

Question

I\'ve downloaded from a supposedly serious source a sage script. It doesn\'t work on my computer, and a quick debugging showed that a problem came from the fact that at some

Answer 1

I would suggest subclassing e.g. collections.abc.MutableSequence for something like this, because once the protocol (in this case: __getitem__, __setitem__, __delitem__, __len__, insert) is implemented all list methods should work on the custom sequence type.

The solution I came up with uses collections.abc.MutableSequence with a list wrapper (_lst) and a helper class component that doesn't know much about anything except that it is subscriptable, i.e. it implements __getitem__ which handles the index modification.

import collections.abc

class _IndexComponent:
    def __getitem__(self, index):
        if index == 0: raise IndexError("Index 0 is a lie.")
        if index > 0: return index -1
        else: return index
        
class OneList(collections.abc.MutableSequence):

    def __init__(self, init: list = None) -> None:
        self._index_comp = _IndexComponent()
        self._lst = []
        if not init is None: # and isinstance(init, list)?
            self._lst.extend(init)
    
    def __getitem__(self, index: int) -> any:
        return self._lst[self._index_comp[index]]

    def __setitem__(self, index: int, val: any) -> None:
        self._lst[self._index_comp] = val

    def __delitem__(self, index: int) -> None:
        del self._lst[self._index_comp[index]]

    def __len__(self) -> int:
        return len(self._lst)

    def insert(self, index: int, val: any) -> None:
        self._lst.insert(self._index_comp[index], val)

    def __repr__(self) -> str:
        return f"{self._lst}"

Now for example pop works although it isn't explicitly implemented:

ol = OneList([1,2,3,4])
print(ol.pop(1))
ol.pop(0) # IndexError

Somehow this feels kind of messy though, I would appriciate if someone shared a better solution.

l = [] l.extend([]) print(l)