Immutable list in Python

Question

I'm trying to make a list which is used throughout the application immutable. I thought wrapping this list in a tuple would do the trick, but it seems that tuple(list) doesn't actually wrap, but copies the list elements.

>>> a = [1, 2, 3, 4]
>>> b = tuple(a)
>>> b
(1, 2, 3, 4)
>>> a[0] = 2
>>> b # was hoping b[0] to be 2
(1, 2, 3, 4)

Is there an easy way of creating a list-backed "view" on this list that is immutable (wrt. operations on this view), but reflects any change that happened to the backing list?

I realise that this question has been asked before, but none of the responses address this view-backing list relationship (in fact some of the comments even suggest that tuples work the way I was hoping they do, but the above snippet suggests otherwise).

Answer 1

If you don't want to copy the data, and want to pass an unchangeable "list" around, one way to do so is to create a proxy object, copy of a list, which disables all changing methods, and refer the reading methods to the original list - something along:

from collections import UserList

class ReadOnlyList(UserList):
    def __init__(self, original):
        self.data = original
    def insert(self, index=None, value=None):
        raise TypeError()
    __setitem__ = insert
    __delitem__ = insert
    append = insert
    extend = insert
    pop = insert
    reverse = insert
    sort = insert

By subclassing "UserList" one ensures all code dealing with the list data will go through the publicly exposed Python methods, and better yet, all the remaining methods are already implemented and proxy to the internal data attribute.

bellow, the original answer from 2014, focusing on Python 2

    class ReadOnlyList(list):
        def __init__(self, other):
            self._list = other
        
        def __getitem__(self, index):
            return self._list[index]
        
        def __iter__(self):
            return iter(self._list)
        
        def __slice__(self, *args, **kw):
            return self._list.__slice__(*args, **kw)
        
        def __repr__(self):
            return repr(self._list)

        def __len__(self):
            return len(self._list)
        
        def NotImplemented(self, *args, **kw):
            raise ValueError("Read Only list proxy")
        
        append = pop = __setitem__ = __setslice__ = __delitem__ = NotImplemented

And, of course, implement whatever other methods you judge necessary, either raising the error (or ignoring the writting instruction) - or acessing the corresponding object in the internal list.