How to create a pointer to a list, such that accessing via the pointer gives immutable elements?

Question

Assume

my_list = [[1], [2], [3], ["a"], [5]]

I want

my_imm = ImmutableList(mylist)

such that

my_imm[2][0] = 0

would either throw an error, or not change my_list[2].

Is there such a construct without copying all the data?

Also assume my_list is long, and I wouldn't want a conversion, but rather an lazy iterator.

Notice this is not a duplicate of this, because I want THE ELEMENTS to be immutable.

Tuples won't solve that out of the box.

do you have access to numpy? https://stackoverflow.com/questions/5541324/immutable-numpy-array — Andrew Pye, Oct 21 '20 at 13:38
numpy accepts strings tho... does require that if you have list of lists, that all have same dimensions. otherwise the nested list will be mutable — Andrew Pye, Oct 21 '20 at 13:40
@AndrewPye Doesn't numpy copy everything on construction? I have the data input as a list. — Gulzar, Oct 21 '20 at 13:45

score 1 · Accepted Answer · answered Oct 21 '20 at 13:59

1

Use a generator to get the elemets as tuples:

my_imm = (tuple(e) for e in my_list)

answered Oct 21 '20 at 13:59

pbacterio

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12

smaller is better (and you got there before I did) +1 – Gulzar Oct 21 '20 at 14:04
but doesn't this still create a copy? – Andrew Pye Oct 21 '20 at 14:07
Kind off. It creates tuples with references to the values. – pbacterio Oct 21 '20 at 14:30

score 0 · Answer 2 · answered Oct 21 '20 at 14:03

from collections.abc import Iterable
from typing import Iterator


class MyImm(Iterable):
    def __init__(self, iterable: Iterable):
        self._iterator = iter(iterable)

    def __next__(self) -> Iterator:
        return tuple(next(self._iterator))

    def __iter__(self):
        return self


my_list = [[1], [2], [3], ["a"], [5]]
my_imm = MyImm(my_list)

for i, e in enumerate(my_imm):
    print(my_list)
    if i == 2:
        e[0] = 0

print(my_list)

output:

"C:/code/EPMD/Kodex/Applications/EPMD-Software/LocationService/ablation/ssss.py",
line 22, in <module>
   e[0] = 0 TypeError: 'tuple' object does not support item assignment [[1], [2], [3], ['a'], [5]] [[1], [2], [3], ['a'], [5]]
[[1], [2], [3], ['a'], [5]]

How to create a pointer to a list, such that accessing via the pointer gives immutable elements?

2 Answers2