Add a generic immutable sequence wrapper class

In anticipation of satisfying #1398, this adds a generic immutable sequence
wrapper class. The idea being that it can be used to wrap up a list or
similar, that you don't want the caller to modify.

This commit aims to get the basics down for this, and also adds a minimal
set of unit tests.

src/textual/_collections.py

@@ -0,0 +1,65 @@
+"""Provides collection-based utility code."""
+
+from __future__ import annotations
+from typing import Generic, TypeVar, Iterator, overload, Iterable
+
+T = TypeVar("T")
+
+
+class ImmutableSequence(Generic[T]):
+    """Class to wrap a sequence of some sort, but not allow modification."""
+
+    def __init__(self, wrap: Iterable[T]) -> None:
+        """Initialise the immutable sequence.
+
+        Args:
+            wrap (Iterable[T]): The iterable value being wrapped.
+        """
+        self._list = list(wrap)
+
+    @overload
+    def __getitem__(self, index: int) -> T:
+        ...
+
+    @overload
+    def __getitem__(self, index: slice) -> ImmutableSequence[T]:
+        ...
+
+    def __getitem__(self, index: int | slice) -> T | ImmutableSequence[T]:
+        return (
+            self._list[index]
+            if isinstance(index, int)
+            else ImmutableSequence[T](self._list[index])
+        )
+
+    def __iter__(self) -> Iterator[T]:
+        return iter(self._list)
+
+    def __len__(self) -> int:
+        return len(self._list)
+
+    def __length_hint__(self) -> int:
+        return len(self)
+
+    def __bool__(self) -> bool:
+        return bool(len(self))
+
+    def __contains__(self, item: T) -> bool:
+        return item in self._list
+
+    def index(self, item: T) -> int:
+        """Return the index of the given item.
+
+        Args:
+            item (T): The item to find in the sequence.
+
+        Returns:
+            int: The index of the item in the sequence.
+
+        Raises:
+            ValueError: If the item is not in the sequence.
+        """
+        return self._list.index(item)
+
+    def __reversed__(self) -> Iterator[T]:
+        return reversed(self._list)

tests/test_collections.py

@@ -0,0 +1,81 @@
+import pytest
+
+from typing import Iterable
+from textual._collections import ImmutableSequence
+
+def wrap(source: Iterable[int]) -> ImmutableSequence[int]:
+    """Wrap an itertable of integers inside an immutable sequence."""
+    return ImmutableSequence[int](source)
+
+
+def test_empty_immutable_sequence() -> None:
+    """An empty immutable sequence should act as anticipated."""
+    assert len(wrap([])) == 0
+    assert bool(wrap([])) is False
+    assert list(wrap([])) == []
+
+
+def test_non_empty_immutable_sequence() -> None:
+    """A non-empty immutable sequence should act as anticipated."""
+    assert len(wrap([0])) == 1
+    assert bool(wrap([0])) is True
+    assert list(wrap([0])) == [0]
+
+
+def test_immutable_sequence_from_empty_iter() -> None:
+    """An immutable sequence around an empty iterator should act as anticipated."""
+    assert len(wrap([])) == 0
+    assert bool(wrap([])) is False
+    assert list(wrap(iter([]))) == []
+
+
+def test_immutable_sequence_from_non_empty_iter() -> None:
+    """An immutable sequence around a non-empty iterator should act as anticipated."""
+    assert len(wrap(range(23))) == 23
+    assert bool(wrap(range(23))) is True
+    assert list(wrap(range(23))) == list(range(23))
+
+
+def test_no_assign_to_immutable_sequence() -> None:
+    """It should not be possible to assign into an immutable sequence."""
+    tester = wrap([1,2,3,4,5])
+    with pytest.raises(TypeError):
+        tester[0] = 23
+    with pytest.raises(TypeError):
+        tester[0:3] = 23
+
+
+def test_no_del_from_iummutable_sequence() -> None:
+    """It should not be possible delete an item from an immutable sequence."""
+    tester = wrap([1,2,3,4,5])
+    with pytest.raises(TypeError):
+        del tester[0]
+
+
+def test_get_item_from_immutable_sequence() -> None:
+    """It should be possible to get an item from an immutable sequence."""
+    assert wrap(range(10))[0] == 0
+    assert wrap(range(10))[-1] == 9
+
+def test_get_slice_from_immutable_sequence() -> None:
+    """It should be possible to get a slice from an immutable sequence."""
+    assert list(wrap(range(10))[0:2]) == [0,1]
+    assert list(wrap(range(10))[0:-1]) == [0,1,2,3,4,5,6,7,8]
+
+
+def test_immutable_sequence_contains() -> None:
+    """It should be possible to see if an immutable sequence contains a value."""
+    tester = wrap([1,2,3,4,5])
+    assert 1 in tester
+    assert 11 not in tester
+
+
+def test_immutable_sequence_index() -> None:
+    tester = wrap([1,2,3,4,5])
+    assert tester.index(1) == 0
+    with pytest.raises(ValueError):
+        _ = tester.index(11)
+
+
+def test_reverse_immutable_sequence() -> None:
+    assert list(reversed(wrap([1,2]))) == [2,1]